Valid Parentheses_Leetcode_#20

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1.题目
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.

2 解法
时间复杂度O(N)

public class Solution {    public boolean isValid(String s) {        ArrayList<Character> arr = new ArrayList<Character>();        for (int i = 0; i < s.length(); i++) {            char ch = s.charAt(i);            if (ch == '(' || ch == '{' || ch == '[') {                arr.add(ch);            }else if (ch == ')' && !arr.isEmpty() && arr.get(arr.size() - 1) == '(') {                arr.remove(arr.size() - 1);            }else if (ch == '}' && !arr.isEmpty() && arr.get(arr.size() - 1) == '{') {                arr.remove(arr.size() - 1);            }else if (ch == ']' && !arr.isEmpty() && arr.get(arr.size() - 1) == '[') {                arr.remove(arr.size() - 1);            }else {                return false;            }        }        return arr.isEmpty() ? true : false;    }}

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