算法:Valid Parentheses(有效的括号)

说明

算法：Valid Parentheses
LeetCode地址：https://leetcode.com/problems/valid-parentheses/

题目：
Given a string containing just the characters ‘(’, ‘)’, ‘{’, ‘}’, ‘[’ and ‘]’, determine if the input string is valid.

An input string is valid if:

Open brackets must be closed by the same type of brackets.
Open brackets must be closed in the correct order.
Note that an empty string is also considered valid.

Example 1:

Input: "()"
Output: true

Example 2:

Input: "()[]{}"
Output: true

Example 3:

Input: "(]"
Output: false

Example 4:

Input: "([)]"
Output: false

Example 5:

Input: "{[]}"
Output: true

解题思路

题意是判断括号匹配是否正确，很明显，我们可以用栈来解决这个问题，当出现左括号的时候入栈，当遇到右括号时，
判断栈顶的左括号是否何其匹配，不匹配的话直接返回 false 即可，最终判断是否空栈即可，这里我们可以
用数组模拟栈的操作使其操作更快，有个细节注意下 top = 1;，从而省去了之后判空的操作和 top - 1 导致数组越界的错误。
时间复杂度为 O(N)。

代码实现

public class ValidParentheses {
    public static boolean isValid(String s) {
        char stack[] = new char[s.length() + 1];
        int top = 1;
        for (char charItem: s.toCharArray()) {
            if (charItem == '(' || charItem == '[' || charItem == '{') {
                stack[top++] = charItem;
            } else if (charItem == ')' && stack[--top] != '(') {
                return false;
            } else if (charItem == ']' && stack[--top] != '[') {
                return false;
            } else if (charItem == '}' && stack[--top] != '{') {
                return false;
            }
        }


        return top == 1;
    }

    public static void main(String[] args) {
        String input1 = "()";
        System.out.println("input: " + input1 + " result: " + isValid(input1));
        String input2 = "()[]{}";
        System.out.println("input: " + input2 + " result: " + isValid(input2));
        String input3 = "(]";
        System.out.println("input: " + input3 + " result: " + isValid(input3));
        String input4 = "([)]";
        System.out.println("input: " + input4 + " result: " + isValid(input4));
        String input5 = "{[]}";
        System.out.println("input: " + input5 + " result: " + isValid(input5));
    }
}

运行结果

input: () result: true
input: ()[]{} result: true
input: (] result: false
input: ([)] result: false
input: {[]} result: true

代码执行效率

Runtime: 2 ms, faster than 100.00% of Java online submissions for Valid Parentheses.
Memory Usage: 37.2 MB, less than 18.74% of Java online submissions for Valid Parentheses.

总结

考查栈的运用，小心栈越界问题。

代码下载：
https://github.com/zgpeace/awesome-java-leetcode/blob/master/code/LeetCode/src/ValidParentheses.java