[LeetCode] 20. 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.

解法一: stack

class Solution {
public:
    bool isValid(string s) {
        unordered_set<char> left = {'(', '[', '{' };
        unordered_set<char> right = {')', ']', '}' };
        unordered_map<char, char> ht = {{'(',')'}, {'[', ']'}, {'{','}'} };
        stack<char> stk;
        bool ret = false;
        
        for (int i = 0; i < s.size(); i++){
            if (left.count(s[i])){
                stk.push(s[i]);
            }else if(right.count(s[i])){
                // check empty !!
                if (!stk.empty() && ht[stk.top()] == s[i]){
                    stk.pop();
                }else{
                    return false;
                }
            }else{
                return false;
            }
        }
        
        return stk.empty();
    }
};

博主在Linked-in面试中碰到过这題的简化版: 一个字符串只包含小括号 '(' 或者')', 检查括号匹配是否正确. 

该简化版的解法, 可以直接用一个计数器, 而不需要堆栈:

bool matched(String s){
    int left = 0;
        
    for (int i = 0; i < s.size(); i++){
        if (s[i] == '('){
            left++;
            continue;
        }else if (s[i] == ')'){
            if (left > 0){
                left--;
            }else{
                return false;
            } 
        }
    }
    
    return left == 0;
 }