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.

Solution:

If we find a left parenthesis, we push its right parenthesis into the stack.

If we find a non-left parenthesis character, we pop the stack and check if there's a match. If not, simply return false.



Code:

public class Solution {
    public boolean isValid(String s) {
        Stack<Character> stack = new Stack<>();
        
        for (char c : s.toCharArray()) {
            if (c == '(') {
                stack.push(')');
            }
            else if (c == '[') {
                stack.push(']');
            }
            else if (c == '{') {
                stack.push('}');
            }
            else if (stack.isEmpty() || stack.pop() != c) {
                return false;
            }
        }
        
        return stack.isEmpty();
    }
}