156.上下翻转二叉树

方法：自底向上迭代

/*** 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 {public TreeNode upsideDownBinaryTree(TreeNode root) {if(root == null || root.left == null && root.right == null){return root;}Stack<TreeNode> stack = new Stack<>(); TreeNode tmp = root;while(tmp != null){stack.push(tmp);tmp = tmp.left; //一直找到最左的节点}TreeNode newRoot = stack.pop();while(!stack.isEmpty()){TreeNode node = stack.pop();TreeNode preLeft = node.left,preRight = node.right;preLeft.left = preRight;preLeft.right = node;node.left = null;node.right = null;}return newRoot;}
}

方法二：递归

/*** 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 {public TreeNode upsideDownBinaryTree(TreeNode root) {if(root == null || root.left == null && root.right == null){return root;}TreeNode left = root.left,right = root.right;TreeNode newRoot = upsideDownBinaryTree(left);left.left = right;left.right = root;root.left = null;  //将root的左右节点都设为空，表示翻转之和root的左右节点不再是left和rightroot.right = null;return newRoot;}
}