Problem:

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.

Solution:

class Solution {
public:
    bool isValid(string s) {
        stack<char> stk;
        for(int i=0; i<s.size(); i++){
            if(isLeft(s[i]))
                stk.push(s[i]);
            else{
                if(stk.empty() || !isClose(stk.top(), s[i])){
                    return false;
                }
                stk.pop();
            }
        }
        return stk.empty();
    }
    
    bool isLeft(char c){
        if(c=='(' || c=='{' || c=='[')
            return true;
        return false;
    }
    
    bool isClose(char l, char r){
        if(l=='(' && r ==')') return true;
        else if(l=='{' && r=='}') return true;
        else if(l=='[' && r==']') return true;
        return false;
    }
};

Analysis:

Time complexity: O(n)
Space complexity: O(n)