LeetCode刷题笔录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.

这题只要求括号是合法的。用一个stack，遇到左括号就丢到stack里，遇到右括号就弹出stack顶上的括号，看看是不是一对。是的话就继续，不是的话返回false。扫描完一遍以后，stack如果不是空的，说明左括号多了，返回false。

public class Solution {
    public boolean isValid(String s) {
        //this stack stores all the left parentheses
        Stack<Character> stack = new Stack<Character>();
        for(int i = 0; i < s.length(); i++){
            char current = s.charAt(i);
            //if the next character is a right par
            if(isLeft(current) == false){
                if(stack.isEmpty()){
                    return false;
                }
                //check the top par on the stack if it formsa pair with the current par
                char top = stack.pop();
                if(isPair(top, current) == false){
                    return false;
                }
            }
            //if the next character is a left par, push onto the stack
            else{
                stack.push(current);
            }
        }
        if(stack.isEmpty() == false)
            return false;
        return true;
        
    }
    
    public boolean isLeft(char c){
        if(c == '[' || c == '{' || c == '(')
            return true;
        else
            return false;
    }
    
    public boolean isPair(char left, char right){
        if(left == '(' && right == ')')
            return true;
        if(left == '[' && right == ']')
            return true;
        if(left == '{' && right == '}')
            return true;
        return false;
    }
}