# 20. Valid Parentheses

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

# Solution

Approach 1: Stacks.

# Code (Python)

Approach 1:

# Code (C++)

Approach 1:

class Solution {
public:
    bool isValid(string s) {
        stack<char> st;
        for (int i = 0; i < s.size(); ++i)
        {
            if (s[i] == '(' || s[i] == '{' || s[i] == '[')
                st.push(s[i]);
            else if (st.empty() || // Need to check if the stack is empty.
                (s[i] == ')' && st.top() != '(') ||
                (s[i] == '}' && st.top() != '{') ||
                (s[i] == ']' && st.top() != '['))
                return false;
            else
                st.pop();
        }
        return st.empty(); // Need to check if the stack is empty.
    }
};

class Solution {
public:
    bool isValid(string s) {
        stack<char> st;
        charMap *umap = new charMap();        
        for (int i = 0; i < s.size(); ++i)
        {
            if (umap->umap.find(s[i]) != umap->umap.end())
                st.push(s[i]);
            else if (st.empty() ||  // Need to check if the stack is empty.
                umap->umap[st.top()] != s[i]) // Use st.top() as the index.
                return false;
            else
                st.pop();
        }
        return st.empty(); // Need to check if the stack is empty.
    }

private:
    struct charMap
    {
        unordered_map<char,char> umap;
        charMap()
        {
            umap['('] = ')';
            umap['{'] = '}';
            umap['['] = ']';
        }
    };
};