20. Valid Parentheses

題目敘述

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

An input string is valid if:

1.Open brackets must be closed by the same type of brackets.
2.Open brackets must be closed in the correct order.
3.Every close bracket has a corresponding open bracket of the same type.

Example 1:
Input: s = "()"
Output: true

Example 2:
Input: s = "()[]{}"
Output: true

Example 3:
Input: s = "(]"
Output: false

程式碼(C++):

class Solution {
public:
    bool isValid(string s) {
        stack<char> st; 
        for (char c : s) 
        { 
            if (c == '(' || c == '{' || c == '[') 
            { 
                st.push(c); 
            } 
            else 
            { 
                if (st.empty() || 
                    (c == ')' && st.top() != '(') || 
                    (c == '}' && st.top() != '{') ||
                    (c == ']' && st.top() != '[')) 
                {
                    return false; 
                }
                st.pop(); 
            }
        }
            return st.empty(); 
    }
};