226. Invert Binary Tree

Invert a binary tree.

Example:

Input:

     4
   /   
  2     7
 /    / 
1   3 6   9

Output:

     4
   /   
  7     2
 /    / 
9   6 3   1

//Time: O(n), Space: O(h)/O(logn) 
//Approach 1: DFS Top down recursive
   public TreeNode invertTree(TreeNode root) {
        if (root == null) {
            return null;
        }
        
        TreeNode temp = root.left;
        root.left = invertTree(root.right);
        root.right = invertTree(temp);
        return root;
    }

//Approach 2: DFS Bottom up recursive(Divide and Concur)
    public TreeNode invertTree(TreeNode root) {
        if (root == null) {
            return null;
        }
        
        TreeNode left = invertTree(root.left);
        TreeNode right = invertTree(root.right);
        root.left = right;
        root.right = left;
        return root;
    }
//Approach3: BFS Queue, 省略