20. 括号的合法性 Valid Parentheses

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.

题意：判断字符串中的括号组合是否合法

解法：使用用栈，如果新字符与栈顶字符成对执行POP，不成对执行PUSH。最后判断Count是否为零

public class Solution {
    public bool IsValid(string s) {
		Stack<char> stack = new Stack<char>();
		foreach(var c in s){
			if (stack.Count == 0) {
				stack.Push(c);
			} else {
				if (!IsPair(stack.Peek(), c)) {
					stack.Push(c);
				} else {
					stack.Pop();
				}
			}
		}
		return stack.Count == 0; 
    }
    
	public bool IsPair(char c1,char c2) {
		if ((c1 == '(' && c2 == ')') || (c1 == ')' && c2 == '(')) {
			return true;
		}else if ((c1 == '{' && c2 == '}') || (c1 == '}' && c2 == '{')) {
			return true;
		}else if ((c1 == '[' && c2 == ']') || (c1 == ']' && c2 == '[')) {
			return true;
		}
		return false;
	}
}

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