20. Valid Parentheses

Valid Parenthese

有效的圆括号

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.

ps：题意很简单 使用stack 依次检测即可

public class Solution {
public boolean isValid(String s) {
Stack<Character> stack = new Stack<Character>();
// Iterate through string until empty
for(int i = 0; i<s.length(); i++) {
// Push any open parentheses onto stack
if(s.charAt(i) == '(' || s.charAt(i) == '[' || s.charAt(i) == '{')
stack.push(s.charAt(i));
// Check stack for corresponding closing parentheses, false if not valid
else if(s.charAt(i) == ')' && !stack.empty() && stack.peek() == '(')  //stack.peek()是用来检测栈顶的元素
stack.pop();
else if(s.charAt(i) == ']' && !stack.empty() && stack.peek() == '[')
stack.pop();
else if(s.charAt(i) == '}' && !stack.empty() && stack.peek() == '{')
stack.pop();
else
return false;
}
// return true if no open parentheses left in stack
return stack.empty();  //此处注意不能直接返回true
}
}