20.valid-parentheses.cpp

/*
 * @lc app=leetcode id=20 lang=cpp
 *
 * [20] Valid Parentheses
 *
 * https://leetcode.com/problems/valid-parentheses/description/
 *
 * algorithms
 * Easy (36.12%)
 * Total Accepted:    545.2K
 * Total Submissions: 1.5M
 * Testcase Example:  '"()"'
 *
 * Given a string containing just the characters '(', ')', '{', '}', '[' and
 * ']', determine if the input string is valid.
 * 
 * An input string is valid if:
 * 
 * 
 * Open brackets must be closed by the same type of brackets.
 * 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
 * 
 * 
 */
#include<bits/stdc++.h>
using namespace std;
class Solution {
public:
    bool isValid(string s) {
        stack<int> st;
        unordered_map<char,char> closer({{')','('}, {']','['}, {'}','{'}});

        for(int i=0;i<s.length();i++){
            if(s[i]=='('|| s[i]=='{' ||s[i]=='[')
                st.emplace(s[i]);
            else if(!st.empty() && st.top()==closer[s[i]])
                st.pop();
            else
                return false;
        }
        return true; 
    }
};