level travel bst

README.md

@@ -34,7 +34,10 @@
 
 [重建二叉树问题](https://www.cnblogs.com/greyzeng/p/16715432.html)
 
-[二叉树的按层遍历](https://www.cnblogs.com/greyzeng/p/16356829.html)
+[二叉树的按层遍历相关问题](https://www.cnblogs.com/greyzeng/p/16356829.html)
+- 类似问题一： 二叉树自底向上层序遍历
+
+    
 
 ## 更多
 

src/main/java/git/snippet/tree/LeetCode_0107_BinaryTreeLevelOrderTraversalII.java

 package git.snippet.tree;
 
-import java.util.ArrayList;
-import java.util.LinkedList;
-import java.util.List;
-import java.util.Queue;
+import java.util.*;
 
 // 笔记：https://www.cnblogs.com/greyzeng/p/16356829.html
 // https://leetcode.com/problems/binary-tree-level-order-traversal-ii
 // 按层遍历进阶：从底部开始遍历
-// Given a binary tree, return the bottom-up level order traversal of its nodes' values. (ie, from
-// left to right, level by level from leaf to root).
-//
-// For example:
-// Given binary tree [3,9,20,null,null,15,7],
-// 3
-// / \
-// 9 20
-// / \
-// 15 7
-// return its bottom-up level order traversal as:
-// [
-// [15,7],
-// [9,20],
-// [3]
-// ]
+// 只需要把按层遍历的队列结构改成栈就可以
 public class LeetCode_0107_BinaryTreeLevelOrderTraversalII {
 
     public List<List<Integer>> levelOrderBottom(TreeNode root) {
-        List<List<Integer>> ans = new ArrayList<>();
-        if (root == null) {
-            return ans;
+        if (null == root) {
+            return new ArrayList<>();
         }
-        TreeNode cur = root;
+        Deque<List<Integer>> stack = new ArrayDeque<>();
+        List<List<Integer>> result = new ArrayList<>();
+        List<Integer> levelRecords = new ArrayList<>();
         TreeNode curEnd = root;
         TreeNode nextEnd = null;
         Queue<TreeNode> queue = new LinkedList<>();
-        queue.offer(cur);
-        List<Integer> items = new LinkedList<>();
+        queue.offer(root);
         while (!queue.isEmpty()) {
-            cur = queue.poll();
-            if (cur.left != null) {
-                queue.offer(cur.left);
-                nextEnd = cur.left;
+            TreeNode poll = queue.poll();
+            levelRecords.add(poll.val);
+            if (null != poll.left) {
+                queue.offer(poll.left);
+                nextEnd = poll.left;
             }
-            if (cur.right != null) {
-                queue.offer(cur.right);
-                nextEnd = cur.right;
+            if (null != poll.right) {
+                queue.offer(poll.right);
+                nextEnd = poll.right;
             }
-            items.add(0, cur.val);
-            if (curEnd == cur) {
+            if (poll == curEnd) {
                 curEnd = nextEnd;
-                // 如果是顺序按层遍历，这段代码只需要改成
-                // ans.add(items);
-                ans.add(0, items);
-                items = new LinkedList<>();
+                stack.push(levelRecords);
+                levelRecords = new ArrayList<>();
             }
         }
-        return ans;
+        while (!stack.isEmpty()) {
+            result.add(stack.poll());
+        }
+        return result;
     }
 
     public class TreeNode {