1. Given two strings, one is derived by adding some characters into the other. Find all these characters. Example: A='abc', B='aabcde', return ['a','d','e']
2. Given an n×n matrix, each element is either 1 or 0. Find the largest sub-matrix that has 1s at all its corners. Time complexity requirement: O(n^3) Example: M=[0,1,1;0,0,1;0,1,1], return [[0,1],[2,2]] (the left-top and right-bottom corners).
