題目：

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.

程式碼：

#include<iostream>
#include<set>
#include<map>
#include<stack>
using namespace std;

bool isValid(string s) {

	int len = s.size();
	if (len == 0)return true;
	if (len % 2 != 0)return false;
	stack<char> cstack;
	map<char, char> m;
	m['('] = ')';
	m['{'] = '}';
	m['['] = ']';
	set<char> left = { '(','{','[' };
	if (!left.count(s[0]))return false;
	cstack.push(s[0]); char last = s[0];

	for (int i = 1; i < len; i++) {
		if (left.count(s[i])) {
			cstack.push(s[i]);
			last = s[i]; }
		else if (s[i] == m[last]) { 
			cstack.pop(); 
			if (cstack.size() == 0)last='0';
			else last = cstack.top(); }
		else return false;
	}

	if (cstack.size() != 0)return false;
	return true;
}
int  main() {
	string s = "()[]{}";
	bool m = isValid(s);
	cout << m << endl;

	return 0;
}