Add README for Valid Parentheses Problem

Author: khairalanam

L-B/0022 ValidParentheses/README.md

@@ -0,0 +1,72 @@
+# Valid Parentheses (L-B)
+
+## Problem
+
+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.
+- Every close bracket has a corresponding open bracket of the same type.
+
+## Testcases
+
+- Input: `"()"` <br>
+  Output: `true`
+
+- Input: `"()[]{}"` <br>
+  Output: `true`
+
+- Input: `"(]"` <br>
+  Output: `false`
+
+## Solution
+
+```javascript
+const ValidParentheses = (s) => {
+  var stack = [];
+
+  obj = {
+    "(": ")",
+    "{": "}",
+    "[": "]",
+  };
+
+  for (let i of s) {
+    if (Object.keys(obj).includes(i)) {
+      stack.push(i);
+    } else if (stack && obj[stack[stack.length - 1]] == i) {
+      stack.pop();
+    } else {
+      return false;
+    }
+  }
+
+  return stack.length === 0 ? true : false;
+};
+```
+
+## How it works
+
+- We initialize a `stack` to keep track of the parentheses.
+- We make an object `obj` which keeps the values for the corresponding opening and closing brackets.
+- The function runs a for loop for the string `s`.
+- If the current letter is part of the `obj`'s keys, then push the letter to the `stack`.
+- Else if the stack is not empty and the top of the `stack` corresponds with the current letter, then pop an element out of the `stack`.
+- Else, return `false`
+- If the for loop terminates normally then return `true` if the stack is empty or else `false`.
+
+## References
+
+- [LeetCode](https://leetcode.com/problems/valid-parentheses/)
+
+## Problem Added By
+
+- [GitHub](https://www.github.com/khairalanam)
+- [LinkedIn](https://www.linkedin.com/in/khair-alanam-b27b69221/)
+- [Twitter](https://twitter.com/khair_alanam)
+
+## Contributing
+
+Pull requests are welcome. For major changes, please open an issue first to discuss what you would like to change.