题解 | #翻转牛群结构#

import java.util.*;

/*
 * public class TreeNode {
 *   int val = 0;
 *   TreeNode left = null;
 *   TreeNode right = null;
 *   public TreeNode(int val) {
 *     this.val = val;
 *   }
 * }
 */

public class Solution {
    /**
     * 遍历二叉树，交换左右节点
     *
     * 借助栈遍历二叉树，每次都取出栈顶元素交换左右子树，如果当前节点的左子树不为空则入栈，如果当前节点的右子树不为空，则入栈等待遍历交换
     * @param root TreeNode类 
     * @return TreeNode类
     */
    public TreeNode invertTree (TreeNode root) {
        if(root == null){
            return root;
        }
        List<Integer>list = new ArrayList<>();
        Stack<TreeNode> stack = new Stack<>();
        stack.push(root);
        while(!stack.isEmpty()){
            TreeNode node = stack.pop();
            TreeNode temp = node.left;
            node.left = node.right;
            node.right = temp;
            if(node.left!=null){
                stack.push(node.left);
            }
            if(node.right != null){
                stack.push(node.right);
            }
        }
       
        return root;
    }
}