Tasks Details
medium
Given a string, find the length of the longest substring in which every character appears an even number of times.
Task Score
50%
Correctness
100%
Performance
20%
John takes computer security very seriously, so he has picked a very long password to secure his files. However, although the password is very strong, it is also hard to memorize.
John decides to create a new password which is easier to remember. Therefore, it must fulfill certain requirements: he wants each character (digit or letter) to appear in the new password an even number of times (possibly zero). Also, since he is so proud of his original password, he wants the new password to be a contiguous substring of the original password.
John has trouble finding such a substring. Help him by finding, for a given string, the length of the longest substring that fulfills the requirements set out above.
Write a function:
class Solution { public int solution(String S); }
that, given a non-empty string S consisting of N characters, returns the length of the longest contiguous substring (possibly empty) of string S in which every character occurs an even number of times.
For example, given S = "6daa6ccaaa6d", the function should return 8. The longest valid password taken from S is "a6ccaaa6"; it contains four letters a, and two each of the digit 6 and letter c. Any longer substring must contain either five letters a or one letter d. Given S = "abca", the function should return 0 (note that aa is not a contiguous substring of S).
Write an efficient algorithm for the following assumptions:
- the length of string S is within the range [1..100,000];
- S consists only of lowercase letters and digits.
Copyright 2009–2025 by Codility Limited. All Rights Reserved. Unauthorized copying, publication or disclosure prohibited.
Solution
Programming language used Java 21
Time spent on task 13 minutes
Notes
not defined yet
Code: 15:50:53 UTC,
java,
autosave
Code: 15:52:45 UTC,
java,
verify,
result: Passed
123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657
// you can also use imports, for example:
// import java.util.*;
// you can write to stdout for debugging purposes, e.g.
// System.out.println("this is a debug message");
import java.util.HashSet;
import java.util.Set;
class Solution {
public int solution(String S) {
// write your code in Java SE 8
if(S.length() > 100000) return 0;
String maxSubStr = "";
try{
if(isAllCharsEven(S)) return S.length();
maxSubStr = "";
for(int i=0; i<S.length(); i++){
if( maxSubStr.length() > S.length()-i ) break;
for(int j=S.length(); i<j; j--){
String str = S.substring(i, j);
if( maxSubStr.length() > str.length() ) break;
if( str.length()%2 == 1 ) continue;
if( maxSubStr.length() < str.length() && isAllCharsEven(str) ){
maxSubStr = str;
}
}
}
}catch(Exception e){
//e.printStackTrace();
return 0;
}
return maxSubStr.length();
}
public boolean isAllCharsEven(String s) throws Exception{
//Set<Character> set = new HashSet<Character>();
int[] range = new int[35]; //26 + 9
for(int i=0; i<s.length(); i++){
Character ch = s.charAt(i);
if( ch >= 97 && ch <= 122 ){
range[ch-'a'+10]++;
} else if (ch >= 48 && ch <= 57) {
range[ch-'0']++;
} else
throw new Exception("Not allowed letter "+ch);
//if( set.contains(ch) ) set.remove(ch);
//else set.add(ch);
}
for(int i=0; i<range.length; i++) {
if(range[i]%2==1) return false;
}
return true;
}
}
Analysis
User test case 1:
['abcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyz']
Code: 15:55:56 UTC,
java,
verify,
result: Passed
123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657
// you can also use imports, for example:
// import java.util.*;
// you can write to stdout for debugging purposes, e.g.
// System.out.println("this is a debug message");
import java.util.HashSet;
import java.util.Set;
class Solution {
public int solution(String S) {
// write your code in Java SE 8
if(S.length() > 100000) return 0;
String maxSubStr = "";
try{
if(isAllCharsEven(S)) return S.length();
maxSubStr = "";
for(int i=0; i<S.length(); i++){
if( maxSubStr.length() > S.length()-i ) break;
for(int j=S.length(); i<j; j--){
String str = S.substring(i, j);
if( maxSubStr.length() > str.length() ) break;
if( str.length()%2 == 1 ) continue;
if( maxSubStr.length() < str.length() && isAllCharsEven(str) ){
maxSubStr = str;
}
}
}
}catch(Exception e){
//e.printStackTrace();
return 0;
}
return maxSubStr.length();
}
public boolean isAllCharsEven(String s) throws Exception{
//Set<Character> set = new HashSet<Character>();
int[] range = new int[35]; //26 + 9
for(int i=0; i<s.length(); i++){
Character ch = s.charAt(i);
if( ch >= 97 && ch <= 122 ){
range[ch-'a'+10]++;
} else if (ch >= 48 && ch <= 57) {
range[ch-'0']++;
} else
throw new Exception("Not allowed letter "+ch);
//if( set.contains(ch) ) set.remove(ch);
//else set.add(ch);
}
for(int i=0; i<range.length; i++) {
if(range[i]%2==1) return false;
}
return true;
}
}
Code: 15:56:12 UTC,
java,
verify,
result: Passed
123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657
// you can also use imports, for example:
// import java.util.*;
// you can write to stdout for debugging purposes, e.g.
// System.out.println("this is a debug message");
import java.util.HashSet;
import java.util.Set;
class Solution {
public int solution(String S) {
// write your code in Java SE 8
if(S.length() > 100000) return 0;
String maxSubStr = "";
try{
if(isAllCharsEven(S)) return S.length();
maxSubStr = "";
for(int i=0; i<S.length(); i++){
if( maxSubStr.length() > S.length()-i ) break;
for(int j=S.length(); i<j; j--){
String str = S.substring(i, j);
if( maxSubStr.length() > str.length() ) break;
if( str.length()%2 == 1 ) continue;
if( maxSubStr.length() < str.length() && isAllCharsEven(str) ){
maxSubStr = str;
}
}
}
}catch(Exception e){
//e.printStackTrace();
return 0;
}
return maxSubStr.length();
}
public boolean isAllCharsEven(String s) throws Exception{
//Set<Character> set = new HashSet<Character>();
int[] range = new int[35]; //26 + 9
for(int i=0; i<s.length(); i++){
Character ch = s.charAt(i);
if( ch >= 97 && ch <= 122 ){
range[ch-'a'+10]++;
} else if (ch >= 48 && ch <= 57) {
range[ch-'0']++;
} else
throw new Exception("Not allowed letter "+ch);
//if( set.contains(ch) ) set.remove(ch);
//else set.add(ch);
}
for(int i=0; i<range.length; i++) {
if(range[i]%2==1) return false;
}
return true;
}
}
Code: 15:56:24 UTC,
java,
verify,
result: Passed
123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657
// you can also use imports, for example:
// import java.util.*;
// you can write to stdout for debugging purposes, e.g.
// System.out.println("this is a debug message");
import java.util.HashSet;
import java.util.Set;
class Solution {
public int solution(String S) {
// write your code in Java SE 8
if(S.length() > 100000) return 0;
String maxSubStr = "";
try{
if(isAllCharsEven(S)) return S.length();
maxSubStr = "";
for(int i=0; i<S.length(); i++){
if( maxSubStr.length() > S.length()-i ) break;
for(int j=S.length(); i<j; j--){
String str = S.substring(i, j);
if( maxSubStr.length() > str.length() ) break;
if( str.length()%2 == 1 ) continue;
if( maxSubStr.length() < str.length() && isAllCharsEven(str) ){
maxSubStr = str;
}
}
}
}catch(Exception e){
//e.printStackTrace();
return 0;
}
return maxSubStr.length();
}
public boolean isAllCharsEven(String s) throws Exception{
//Set<Character> set = new HashSet<Character>();
int[] range = new int[35]; //26 + 9
for(int i=0; i<s.length(); i++){
Character ch = s.charAt(i);
if( ch >= 97 && ch <= 122 ){
range[ch-'a'+10]++;
} else if (ch >= 48 && ch <= 57) {
range[ch-'0']++;
} else
throw new Exception("Not allowed letter "+ch);
//if( set.contains(ch) ) set.remove(ch);
//else set.add(ch);
}
for(int i=0; i<range.length; i++) {
if(range[i]%2==1) return false;
}
return true;
}
}
Code: 15:59:53 UTC,
java,
verify,
result: Passed
123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657
// you can also use imports, for example:
// import java.util.*;
// you can write to stdout for debugging purposes, e.g.
// System.out.println("this is a debug message");
import java.util.HashSet;
import java.util.Set;
class Solution {
public int solution(String S) {
// write your code in Java SE 8
if(S.length() > 100000) return 0;
String maxSubStr = "";
try{
if(isAllCharsEven(S)) return S.length();
maxSubStr = "";
for(int i=0; i<S.length(); i++){
if( maxSubStr.length() > S.length()-i ) break;
for(int j=S.length(); i<j; j--){
String str = S.substring(i, j);
if( maxSubStr.length() > str.length() ) break;
if( str.length()%2 == 1 ) continue;
if( maxSubStr.length() < str.length() && isAllCharsEven(str) ){
maxSubStr = str;
}
}
}
}catch(Exception e){
//e.printStackTrace();
return 0;
}
return maxSubStr.length();
}
public boolean isAllCharsEven(String s) throws Exception{
//Set<Character> set = new HashSet<Character>();
int[] range = new int[35]; //26 + 9
for(int i=0; i<s.length(); i++){
Character ch = s.charAt(i);
if( ch >= 97 && ch <= 122 ){
range[ch-'a'+10]++;
} else if (ch >= 48 && ch <= 57) {
range[ch-'0']++;
} else
throw new Exception("Not allowed letter "+ch);
//if( set.contains(ch) ) set.remove(ch);
//else set.add(ch);
}
for(int i=0; i<range.length; i++) {
if(range[i]%2==1) return false;
}
return true;
}
}
Code: 16:03:11 UTC,
java,
verify,
result: Passed
123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657
// you can also use imports, for example:
// import java.util.*;
// you can write to stdout for debugging purposes, e.g.
// System.out.println("this is a debug message");
import java.util.HashSet;
import java.util.Set;
class Solution {
public int solution(String S) {
// write your code in Java SE 8
if(S.length() > 100000) return 0;
String maxSubStr = "";
try{
if(isAllCharsEven(S)) return S.length();
maxSubStr = "";
for(int i=0; i<S.length(); i++){
if( maxSubStr.length() > S.length()-i ) break;
for(int j=S.length(); i<j; j--){
String str = S.substring(i, j);
if( maxSubStr.length() > str.length() ) break;
if( str.length()%2 == 1 ) continue;
if( maxSubStr.length() < str.length() && isAllCharsEven(str) ){
maxSubStr = str;
}
}
}
}catch(Exception e){
//e.printStackTrace();
return 0;
}
return maxSubStr.length();
}
public boolean isAllCharsEven(String s) throws Exception{
//Set<Character> set = new HashSet<Character>();
int[] range = new int[35]; //26 + 9
for(int i=0; i<s.length(); i++){
Character ch = s.charAt(i);
if( ch >= 97 && ch <= 122 ){
range[ch-'a'+9]++;
} else if (ch >= 48 && ch <= 57) {
range[ch-'0']++;
} else
throw new Exception("Not allowed letter "+ch);
//if( set.contains(ch) ) set.remove(ch);
//else set.add(ch);
}
for(int i=0; i<range.length; i++) {
if(range[i]%2==1) return false;
}
return true;
}
}
Code: 16:03:21 UTC,
java,
verify,
result: Passed
123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657
// you can also use imports, for example:
// import java.util.*;
// you can write to stdout for debugging purposes, e.g.
// System.out.println("this is a debug message");
import java.util.HashSet;
import java.util.Set;
class Solution {
public int solution(String S) {
// write your code in Java SE 8
if(S.length() > 100000) return 0;
String maxSubStr = "";
try{
if(isAllCharsEven(S)) return S.length();
maxSubStr = "";
for(int i=0; i<S.length(); i++){
if( maxSubStr.length() > S.length()-i ) break;
for(int j=S.length(); i<j; j--){
String str = S.substring(i, j);
if( maxSubStr.length() > str.length() ) break;
if( str.length()%2 == 1 ) continue;
if( maxSubStr.length() < str.length() && isAllCharsEven(str) ){
maxSubStr = str;
}
}
}
}catch(Exception e){
//e.printStackTrace();
return 0;
}
return maxSubStr.length();
}
public boolean isAllCharsEven(String s) throws Exception{
//Set<Character> set = new HashSet<Character>();
int[] range = new int[35]; //26 + 9
for(int i=0; i<s.length(); i++){
Character ch = s.charAt(i);
if( ch >= 97 && ch <= 122 ){
range[ch-'a'+9]++;
} else if (ch >= 48 && ch <= 57) {
range[ch-'0']++;
} else
throw new Exception("Not allowed letter "+ch);
//if( set.contains(ch) ) set.remove(ch);
//else set.add(ch);
}
for(int i=0; i<range.length; i++) {
if(range[i]%2==1) return false;
}
return true;
}
}
Code: 16:03:25 UTC,
java,
final,
score:
50
123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657
// you can also use imports, for example:
// import java.util.*;
// you can write to stdout for debugging purposes, e.g.
// System.out.println("this is a debug message");
import java.util.HashSet;
import java.util.Set;
class Solution {
public int solution(String S) {
// write your code in Java SE 8
if(S.length() > 100000) return 0;
String maxSubStr = "";
try{
if(isAllCharsEven(S)) return S.length();
maxSubStr = "";
for(int i=0; i<S.length(); i++){
if( maxSubStr.length() > S.length()-i ) break;
for(int j=S.length(); i<j; j--){
String str = S.substring(i, j);
if( maxSubStr.length() > str.length() ) break;
if( str.length()%2 == 1 ) continue;
if( maxSubStr.length() < str.length() && isAllCharsEven(str) ){
maxSubStr = str;
}
}
}
}catch(Exception e){
//e.printStackTrace();
return 0;
}
return maxSubStr.length();
}
public boolean isAllCharsEven(String s) throws Exception{
//Set<Character> set = new HashSet<Character>();
int[] range = new int[35]; //26 + 9
for(int i=0; i<s.length(); i++){
Character ch = s.charAt(i);
if( ch >= 97 && ch <= 122 ){
range[ch-'a'+9]++;
} else if (ch >= 48 && ch <= 57) {
range[ch-'0']++;
} else
throw new Exception("Not allowed letter "+ch);
//if( set.contains(ch) ) set.remove(ch);
//else set.add(ch);
}
for(int i=0; i<range.length; i++) {
if(range[i]%2==1) return false;
}
return true;
}
}Analysis summary
The following issues have been detected: timeout errors.
Analysis
Detected time complexity:
O(N**3)
expand all
Correctness tests
1.
0.004 s
OK
2.
0.004 s
OK
3.
0.004 s
OK
4.
0.004 s
OK
5.
0.008 s
OK
6.
0.004 s
OK
1.
0.004 s
OK
2.
0.004 s
OK
3.
0.008 s
OK
4.
0.016 s
OK
5.
0.008 s
OK
expand all
Correctness/performance tests
1.
0.008 s
OK
2.
0.040 s
OK
1.
0.068 s
OK
2.
0.048 s
OK
3.
0.008 s
OK
expand all
Performance tests
random2
random strings, all characters, N <= 500
random strings, all characters, N <= 500
✘
TIMEOUT ERROR
running time: 3.08 sec., time limit: 0.10 sec.
running time: 3.08 sec., time limit: 0.10 sec.
1.
3.080 s
TIMEOUT ERROR,
running time: 3.08 sec., time limit: 0.10 sec.
2.
2.072 s
TIMEOUT ERROR,
running time: 2.07 sec., time limit: 0.10 sec.
long_blocks
long blocks of same letters, N <= 100,000
long blocks of same letters, N <= 100,000
✘
TIMEOUT ERROR
running time: >6.00 sec., time limit: 0.56 sec.
running time: >6.00 sec., time limit: 0.56 sec.
1.
6.000 s
TIMEOUT ERROR,
running time: >6.00 sec., time limit: 0.56 sec.
2.
6.000 s
TIMEOUT ERROR,
running time: >6.00 sec., time limit: 0.56 sec.
most_characters
most characters in substring, N <= 100,000
most characters in substring, N <= 100,000
✘
TIMEOUT ERROR
running time: >6.00 sec., time limit: 0.66 sec.
running time: >6.00 sec., time limit: 0.66 sec.
1.
6.000 s
TIMEOUT ERROR,
running time: >6.00 sec., time limit: 0.66 sec.
2.
6.000 s
TIMEOUT ERROR,
running time: >6.00 sec., time limit: 0.66 sec.
3.
6.000 s
TIMEOUT ERROR,
running time: >6.00 sec., time limit: 0.64 sec.
random3
random strings, all characters, N <= 100,000
random strings, all characters, N <= 100,000
✘
TIMEOUT ERROR
running time: >6.00 sec., time limit: 0.69 sec.
running time: >6.00 sec., time limit: 0.69 sec.
1.
6.000 s
TIMEOUT ERROR,
running time: >6.00 sec., time limit: 0.69 sec.
2.
6.000 s
TIMEOUT ERROR,
running time: >6.00 sec., time limit: 0.69 sec.