題目連結
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

題意：給定一個由括號字元組成的字串，問是否合法。
思路：
最簡單的思路，遍歷判斷入棧，最後判斷棧是否為空。

Code：

import java.util.Stack;

public
 
 class Solution {
    public boolean isValid(String s) {
        Stack<Character> stack = new Stack<Character>();
        for (char ch : s.toCharArray()) {
            if (ch == '(') {
                stack.push(')');
            } else if (ch == '[') {
                stack.push(']');
            } else
 
 if (ch == '{') {
                stack.push('}');
            } else if (stack.empty() || stack.pop() != ch) {///棧為空或者棧頂不等於ch
                return false;
            }
        }
        return stack.empty();
    }

    public static void main(String[] args) {
        String str = "" + "()";
        boolean res = new Solution().isValid(str);
        System.out.println(res);
    }
}