/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    // 层序遍历，遍历过程中对每一个节点的位置进行交换
    public TreeNode invertTree(TreeNode root) {
        if (root == null) {
            return root;
        }
        Queue<TreeNode> que = new LinkedList<>();
        que.offer(root);
        while (!que.isEmpty()) {
            int len = que.size();
            while (len > 0) {
                TreeNode tmp = que.poll();
                TreeNode tmpSon = tmp.left;
                tmp.left = tmp.right;
                tmp.right = tmpSon;
                if (tmp.left != null) {
                    que.offer(tmp.left);
                }
                if (tmp.right != null) {
                    que.offer(tmp.right);
                }
                len--;
            }
        }
        return root;  
    } 
}