Codility-Stacks and Queues-Determine whether a given string of parentheses is properly nested

Brackets.md

A string S consisting of N characters is considered to be properly nested if any of the following conditions is true:

S is empty; S has the form "(U)" or "[U]" or "{U}" where U is a properly nested string; S has the form "VW" where V and W are properly nested strings. For example, the string "{[()()]}" is properly nested but "([)()]" is not.

Write a function:

def solution(S)

that, given a string S consisting of N characters, returns 1 if S is properly nested and 0 otherwise.

For example, given S = "{[()()]}", the function should return 1 and given S = "([)()]", the function should return 0, as explained above.

Assume that:

• N is an integer within the range [0..200,000];

• string S consists only of the following characters: "(", "{", "[", "]", "}" and/or ")". Complexity:

• expected worst-case time complexity is O(N);

• expected worst-case space complexity is O(N) (not counting the storage required for input arguments).

# you can write to stdout for debugging purposes, e.g.
# print "this is a debug message"


def isValidPair(left, right):
    if left == '(' and right == ')':
        return True
    if left == '[' and right == ']':
        return True 
    if left == '{' and right == '}':
        return True   
    return False


def solution(S):
    # write your code in Python 2.7
    stack = []
    
    for symbol in S:
        if symbol == '[' or symbol == '{' or symbol == '(':
            stack.append(symbol)
        else:
            if len(stack) == 0:
                return 0
            last = stack.pop()
            if not isValidPair(last, symbol):
                return 0
     
    if len(stack) != 0:
        return 0
             
    return 1