LeetCode: Valid Parentheses 解题报告

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.

SOLUTION1:

使用stack来解决的简单题目。所有的字符依次入栈

1. 遇到成对的括号弹栈，弹栈不成对返回false.

2. 栈为空只能压入左括号

3. 扫描完成时，栈应该为空，否则返回FALSE.

SOLUTION 2:

使用堆栈解决，比较简单。push右括号时，检查左括号在不在，如果不在，返回false,否则弹出一个左括号。

最后看栈是否为空，不为空代表括号数不对称，也要返回false;

public class Solution {
    public boolean isValid(String s) {
        if (s == null) {
            return false;
        }
        
        int len = s.length();
        
        // bug 1: don't use s as the name of the stack.
        Stack<Character> stk = new Stack<Character>();
        
        for (int i = 0; i < len; i++) {
            char c = s.charAt(i);
            switch(c) {
                case '(':
                case '[':
                case '{':
                    stk.push(c);
                    break;
                case ')':
                    if (!stk.isEmpty() && stk.peek() == '(') {
                        stk.pop();
                    } else {
                        return false;
                    }
                    break;
                case '}':
                    if (!stk.isEmpty() && stk.peek() == '{') {
                        stk.pop();
                    } else {
                        return false;
                    }
                    break;
                case ']':
                    if (!stk.isEmpty() && stk.peek() == '[') {
                        stk.pop();
                    } else {
                        return false;
                    }
                    break;    
            }
        }
        
        return stk.isEmpty();
    }
}

https://github.com/yuzhangcmu/LeetCode_algorithm/blob/master/stack/IsValid.java