20. Valid Parentheses.cpp

/*
Given a string s 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.
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

Constraints:
1 <= s.length <= 104
s consists of parentheses only '()[]{}'.
*/

#include <stack>
class Solution {
public:
    bool isValid(string s) {
    
     stack<char> st;
        
		for(int i=0; i<s.length(); i++){
            if(s[i] == '(' || s[i] == '{' || s[i] == '['){
                st.push(s[i]);
            }
            else{
                if((st.empty()) || (s[i] == ')' && st.top() != '(') || (s[i] == '}' && st.top() != '{') || (s[i] == ']' && st.top() != '[')){
                    return false;
                }
                st.pop();
            }
        }        
        return st.empty();
    }
};