LeetCode（20）——Valid Parentheses

题目：

Given a string containing just the characters '(', ')', '{', '}', '[' and ']', determine if the input string is valid.

An input string is valid if:

1. Open brackets must be closed by the same type of brackets.
2. 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

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AC：

class Solution {
	boolean isValidParentheses(char begin, char end) {
		if (('{' == begin && '}' == end) ||
				('[' == begin && ']' == end) ||
				('(' == begin && ')' == end)){
			return true;
		}
		
		return false;
	}
	
	public boolean isValid(String s) {
        if (null == s || "" == s) {
        	return true;
        }
        
        LinkedList<Character> result = new LinkedList<>();
        
        for (char e : s.toCharArray()) {
        	if (false == result.isEmpty() && true == isValidParentheses(result.getFirst(), e)) {
        		result.removeFirst();
        	}
        	else {
        		result.addFirst(e);
        	}
        }
        
        return true == result.isEmpty() ? true : false;
    }
}

符号要配对，一前一后，一一对应。一对一对的消除的话，最后应该全部消除才是合法。

考虑使用栈，当前符号和栈顶的符号配对的话，则栈顶元素弹出，否则压栈。最后栈空即为合法。