Stacks

Given a string s containing only the characters '(', ')', '{', '}', '[', ']', determine if the input string is valid: open brackets are closed by the same type of brackets, and open brackets are closed in the correct order. Solve using a stack.

Given an array heights where heights[i] is the height of the i-th building in a row and the ocean lies to the right of the last building, a building has an ocean view if every building to its right is strictly shorter than it. Return the indices of all buildings with an ocean view, in increasing order.

Given a string s of lowercase letters and the characters ( and ), remove the minimum number of parentheses (from any positions) so that the resulting string is valid — every ( has a matching ) and every ) has a matching (. Return any valid result string.

Given n non-negative integers representing an elevation map where the width of each bar is 1, compute how many units of water can be trapped after raining.