leetcode-20 Valid Parentheses

Posted 2021-01-17 tingweichen

Given a string 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.

Note that an empty string is also considered valid.

Example 1:

Input: "()"
Output: true

Example 2:

Input: "()[]{}"
Output: true

Example 3:

Input: "(]"
Output: false

Example 4:

Input: "([)]"
Output: false

Example 5:

Input: "{[]}"
Output: true

想法：因为最近在学习栈的使用，因而这题使用栈来解决。遍历字符串，如果匹配到相应的(,{,[.就将对应的).],}压入栈中。

如果没有匹配到上述字符，就出栈，看栈顶元素是否与对应的字符相等。

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