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Description

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.

Brief

括号的匹配问题。

Input

（1）"()"

（2）"()[]{}"

Output

（1）true

（2）true

Explanation

Note that an empty string is also considered valid.

Ideas

栈问题，扫描每个字符串的时候，如果为前括号就放到栈中，如果为后括号，就比较栈顶元素是否匹配，如果匹配则出栈；不匹配返回false。

Code

class Solution {
    public boolean isValid(String s) {
       if (s.length() == 0) {
            return true;
        }
        Stack<Character> stack = new Stack<>();
        for (int i = 0; i < s.length(); i++) {
            if (s.charAt(i) == '(' || s.charAt(i) == '[' || s.charAt(i) == '{') {
                stack.push(s.charAt(i));
            } else if (s.charAt(i) == ')' || s.charAt(i) == ']' || s.charAt(i) == '}') {
                if (stack.size() == 0) {
                    return false;
                }
                if (s.charAt(i) == ')') {
                    if (stack.peek() == '(') {
                        stack.pop();
                    } else {
                        return false;
                    }
                }
            }
            if (s.charAt(i) == ']') {
                if (stack.peek() == '[') {
                    stack.pop();
                } else {
                    return false;
                }
            }
            if (s.charAt(i) == '}') {
                if (stack.peek() == '{') {
                    stack.pop();
                } else {
                    return false;
                }
            }

        }
        if (!stack.isEmpty()) {
            return false;
        }
        return true;
    }
}