Implement stack using array https://www.geeksforgeeks.org/problems/implement-stack-using-array/1

Backspace String Compare 

https://leetcode.com/problems/backspace-string-compare/description/
Solution - https://leetcode.com/problems/backspace-string-compare/submissions/1260650161

Closest smaller on left - 

https://www.geeksforgeeks.org/problems/smallest-number-on-left3403/1

Solution - https://discuss.geeksforgeeks.org/comment/8a1a79cb-fee2-4d7f-a47c-71c7335f6df4/practice

Largest Rectangle in Histogram

https://leetcode.com/problems/largest-rectangle-in-histogram/description/

Solution 1 - https://leetcode.com/problems/largest-rectangle-in-histogram/submissions/1261030550

Solution 2 - https://leetcode.com/problems/largest-rectangle-in-histogram/submissions/1864166546