#435 · #427 · Construct Quad Tree(建立四叉树)

Given an n x n binary matrix grid with n a power of two, construct a quad tree where each node has isLeaf, val (only meaningful for leaves) and four children in the order top-left, top-right, bottom-left, bottom-right. A node is a leaf when all values in its region are identical. Constraints: n == grid.length == grid[i].length, n is a power of two, n <= 64.