Valid Parentheses.cpp

/*
Valid Parentheses
=================

Given a string s 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.

Example 1:
Input: s = "()"
Output: true

Example 2:
Input: s = "()[]{}"
Output: true

Example 3:
Input: s = "(]"
Output: false

Example 4:
Input: s = "([)]"
Output: false

Example 5:
Input: s = "{[]}"
Output: true

Constraints:
1 <= s.length <= 104
s consists of parentheses only '()[]{}'.
*/

class Solution
{
public:
  bool isValid(string s)
  {
    stack<char> st;
    for (auto &i : s)
    {
      if (i == '(' || i == '{' || i == '[')
        st.push(i);
      else
      {
        if (!st.size())
          return false;
        if (i == ')' && st.top() == '(')
          st.pop();
        else if (i == '}' && st.top() == '{')
          st.pop();
        else if (i == ']' && st.top() == '[')
          st.pop();
        else
          return false;
      }
    }
    return st.size() == 0;
  }
};