A non-empty array A consisting of N integers is given. The consecutive elements of array A represent consecutive cars on a road.
Array A contains only 0s and/or 1s:
- 0 represents a car traveling east,
- 1 represents a car traveling west.
The goal is to count passing cars. We say that a pair of cars (P, Q), where 0 ≤ P < Q < N, is passing when P is traveling to the east and Q is traveling to the west.
For example, consider array A such that:
A[0] = 0 A[1] = 1 A[2] = 0 A[3] = 1 A[4] = 1We have five pairs of passing cars: (0, 1), (0, 3), (0, 4), (2, 3), (2, 4).
Write a function:
def solution(A)
that, given a non-empty array A of N integers, returns the number of pairs of passing cars.
The function should return −1 if the number of pairs of passing cars exceeds 1,000,000,000.
For example, given:
A[0] = 0 A[1] = 1 A[2] = 0 A[3] = 1 A[4] = 1the function should return 5, as explained above.
Write an efficient algorithm for the following assumptions:
- N is an integer within the range [1..100,000];
- each element of array A is an integer that can have one of the following values: 0, 1.
# you can write to stdout for debugging purposes, e.g.
# print "this is a debug message"
def solution(A):
# write your code in Python 2.7
L = len(A)
if L == 1:
return 0
for i in range(L-1):
if A[i]: # 1 or west
A[i] = A[i-1] + 1
else: # 0 or east
A[i] = A[i-1]
n = n + A[i]
if n > 1000000000:
return -1
return n
Traceback (most recent call last): File "user.py", line 109, in <module> main() File "user.py", line 86, in main result = sol.solution ( A ) File "/tmp/solution.py", line 16, in solution n = n + A[i] UnboundLocalError: local variable 'n' referenced before assignment
# you can write to stdout for debugging purposes, e.g.
# print "this is a debug message"
def solution(A):
# write your code in Python 2.7
L = len(A)
n = 0
if L == 1:
return 0
for i in range(L-1):
if A[i]: # 1 or west
A[i] = A[i-1] + 1
else: # 0 or east
A[i] = A[i-1]
n = n + A[i]
if n > 1000000000:
return -1
return n
# you can write to stdout for debugging purposes, e.g.
# print "this is a debug message"
def solution(A):
# write your code in Python 2.7
L = len(A)
n = 0
if L == 1:
return 0
for i in range(L-2, -1, -1):
if A[i]: # 1 or west
A[i] = A[i-1] + 1
else: # 0 or east
A[i] = A[i-1]
n = n + A[i]
if n > 1000000000:
return -1
return n
# you can write to stdout for debugging purposes, e.g.
# print "this is a debug message"
def solution(A):
# write your code in Python 2.7
L = len(A)
n = 0
if L == 1:
return 0
for i in range(L-2, -1, -1):
if A[i]: # 1 or west
A[i] = A[i+1] + 1
else: # 0 or east
A[i] = A[i+1]
n = n + A[i]
if n > 1000000000:
return -1
return n
# you can write to stdout for debugging purposes, e.g.
# print "this is a debug message"
def solution(A):
# write your code in Python 2.7
L = len(A)
n = 0
if L == 1:
return 0
for i in range(L-2, -1, -1):
if A[i]: # 1 or west
A[i] = A[i+1] + 1
else: # 0 or east
A[i] = A[i+1]
n = n + A[i]
if n > 1000000000:
return -1
return n
[0, 0, 0, 0, 0, 0, 0, 0, 0, 0]
[0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1]
[1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0]
function result: 0
function result: 10
function result: 0
# you can write to stdout for debugging purposes, e.g.
# print "this is a debug message"
def solution(A):
# write your code in Python 2.7
L = len(A)
n = 0
if L == 1:
return 0
for i in range(L-2, -1, -1):
if A[i]: # 1 or west
A[i] = A[i+1] + 1
else: # 0 or east
A[i] = A[i+1]
n = n + A[i]
if n > 1000000000:
return -1
return n
[0, 0, 0, 0, 0, 0, 0, 0, 0, 0]
[0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1]
[1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0]
function result: 0
function result: 10
function result: 0
# you can write to stdout for debugging purposes, e.g.
# print "this is a debug message"
def solution(A):
# write your code in Python 2.7
L = len(A)
n = 0
if L == 1:
return 0
for i in range(L-2, -1, -1):
if A[i]: # 1 or west
A[i] = A[i+1] + 1
else: # 0 or east
A[i] = A[i+1]
n = n + A[i]
if n > 1000000000:
return -1
return n
The solution obtained perfect score.