Title: Invert Binary Tree Source: leetcode.com Invert a binary tree. 4 / \ 2 7 / \ / \ 1 3 6 9 12345      4   /   \  2     7 / \   / \1   3 6   9 to 4 / \ 7 2 / \ / \ 9 6 3 1 12345      4   /   \  7     ...