LeetCode（49）-Valid Parentheses

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.

思路：

代码：

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