Invert Binary Tree

Invert a binary tree.

Example:

Input:

     4
   /   \
  2     7
 / \   / \
1   3 6   9

Output:

     4
   /   \
  7     2
 / \   / \
9   6 3   1

o(n), o(level);

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public TreeNode invertTree(TreeNode root) {
        
        
        
        if(root == null)
            return null;
        
        
        helper(root);
         return root;
    }
    
    public void helper(TreeNode root){
         if(root == null)
            return;
        
        
        TreeNode tmp = root.left;
        root.left = root.right;
        root.right = tmp;
        
        helper(root.left);
        helper(root.right);
    }
}

BFS 迭代写法

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public TreeNode invertTree(TreeNode root) {
        if(root == null)
            return null;
        
        Queue<TreeNode> queue = new LinkedList<>();
        
        queue.offer(root);
        
        while(!queue.isEmpty()){
            TreeNode node = queue.poll();
            
            TreeNode tmp = node.left;
            
            node.left = node.right;
            
            node.right  = tmp;
            
            if(node.left != null)
                queue.offer(node.left);
            
            if(node.right != null)
                queue.offer(node.right);
        }
        
        
        return root;
    }
}