20.valid-parentheses.cpp

/*
 * [20] Valid Parentheses
 *
 * https://leetcode.com/problems/valid-parentheses/description/
 *
 * algorithms
 * Easy (34.08%)
 * Total Accepted:    336.4K
 * Total Submissions: 986.8K
 * Testcase Example:  '"()"'
 *
 * Given a string containing just the characters '(', ')', '{', '}', '[' and
 * ']', determine if the input string is valid.
 * 
 * An input string is valid if:
 * 
 * 
 * Open brackets must be closed by the same type of brackets.
 * 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
 * 
 * 
 */
class Solution {
public:
    bool isValid(string s) {
	stack<char> st;
	int n = s.size();
	if (!n) return true;
	unordered_map<char, char> um;
	um[')'] = '(';
	um[']'] = '[';
	um['}'] = '{';
	for (auto c : s) {
		if (st.size() && um[c] == st.top()) st.pop();
		else st.push(c);
	}    
	return st.empty();
    }
};