LeetCode 20. Valid Parentheses

Problem:

https://leetcode.com/problems/valid-parentheses/

Given a string containing just the characters '(', ')', '{', '}', '[' and ']', determine if the input string is valid.

The brackets must close in the correct order, "()" and "()[]{}" are all valid but "(]" and "([)]" are not.

 

Thought:

traverse the string, when meet ' ( ' ,' [ ' ,' { '  push it into stack, when meet ' ) ', ' } ' , ' ] ' , pop stack ,campare if they meet each other, if failes ,return false.

if stack is not empty after, return false.

otherwise, return true

 

Code C++:

class Solution {
public:
    bool isValid(string s) {
        if (s.length() % 2) {
            return false;
        }
        
        vector<char> stack;
        stack.push_back(s[0]);
        
        for (int i = 1; i < s.length(); i++) {
            if (s[i] == '[' || s[i] == '{' || s[i] == '(')
                stack.push_back(s[i]);
            else if (s[i] == '}'){
                char c = stack.back();
                stack.pop_back();
                if (c != '{')
                    return false;
            } else if (s[i] == ']'){
                char c = stack.back();
                stack.pop_back();
                if (c != '[')
                    return false;
            } else if (s[i] == ')'){
                char c = stack.back();
                stack.pop_back();
                if (c != '(')
                    return false;
            } else
                return false;
        }
        
        if (stack.size() > 0)
            return false;
        return true;
    }
};