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.

import java.util.Stack;
 
public class Solution {
    public boolean isValid(String s) {
        Stack<Character> stack = new Stack<Character>();
        //使用foreach循环
        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();
    }
}

Java2

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