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Author: BaffinLee

101-200/107. Binary Tree Level Order Traversal II.md

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+# 107. Binary Tree Level Order Traversal II
+
+- Difficulty: Easy.
+- Related Topics: Tree, Breadth-first Search.
+- Similar Questions: Binary Tree Level Order Traversal, Average of Levels in Binary Tree.
+
+## Problem
+
+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]
+]
+```
+
+## Solution 1
+
+```javascript
+/**
+ * Definition for a binary tree node.
+ * function TreeNode(val) {
+ *     this.val = val;
+ *     this.left = this.right = null;
+ * }
+ */
+/**
+ * @param {TreeNode} root
+ * @return {number[][]}
+ */
+var levelOrderBottom = function(root) {
+  var res = [];
+  helper(root, 0, res);
+  return res.reverse();
+};
+
+var helper = function (root, level, res) {
+  if (!root) return;
+  if (!res[level]) res[level] = [];
+  res[level].push(root.val);
+  helper(root.left, level + 1, res);
+  helper(root.right, level + 1, res);
+};
+```
+
+**Explain:**
+
+nope.
+
+**Complexity:**
+
+* Time complexity : O(n).
+* Space complexity : O(n).
+
+## Solution 2
+
+```javascript
+/**
+ * Definition for a binary tree node.
+ * function TreeNode(val) {
+ *     this.val = val;
+ *     this.left = this.right = null;
+ * }
+ */
+/**
+ * @param {TreeNode} root
+ * @return {number[][]}
+ */
+var levelOrderBottom = function(root) {
+  var res = [];
+  helper(root, 0, res);
+  return res;
+};
+
+var helper = function (root, level, res) {
+  if (!root) return;
+  if (res.length < level + 1) res.unshift([]);
+  res[res.length - level - 1].push(root.val);
+  helper(root.left, level + 1, res);
+  helper(root.right, level + 1, res);
+};
+```
+
+**Explain:**
+
+nope.
+
+**Complexity:**
+
+* Time complexity : O(n).
+* Space complexity : O(n).
+
+## Solution 3
+
+```javascript
+/**
+ * Definition for a binary tree node.
+ * function TreeNode(val) {
+ *     this.val = val;
+ *     this.left = this.right = null;
+ * }
+ */
+/**
+ * @param {TreeNode} root
+ * @return {number[][]}
+ */
+var levelOrderBottom = function(root) {
+  var res = [];
+  var stack = [[root, 0]];
+  var level = 0;
+  var node = null;
+  
+  while (stack.length) {
+    [node, level] = stack.pop();
+    if (node) {
+      if (res.length < level + 1) res.unshift([]);
+      res[res.length - level - 1].push(node.val);
+      stack.push([node.right, level + 1]);
+      stack.push([node.left, level + 1]);
+    }
+  }
+  
+  return res;
+};
+```
+
+**Explain:**
+
+nope.
+
+**Complexity:**
+
+* Time complexity : O(n).
+* Space complexity : O(n).
+
+## Solution 4
+
+```javascript
+/**
+ * Definition for a binary tree node.
+ * function TreeNode(val) {
+ *     this.val = val;
+ *     this.left = this.right = null;
+ * }
+ */
+/**
+ * @param {TreeNode} root
+ * @return {number[][]}
+ */
+var levelOrderBottom = function(root) {
+  var res = [];
+  var queue = [[root, 0]];
+  var level = 0;
+  var node = null;
+  
+  while (queue.length) {
+    [node, level] = queue.shift();
+    if (node) {
+      if (res.length < level + 1) res.unshift([]);
+      res[res.length - level - 1].push(node.val);
+      queue.push([node.left, level + 1]);
+      queue.push([node.right, level + 1]);
+    }
+  }
+  
+  return res;
+};
+```
+
+**Explain:**
+
+nope.
+
+**Complexity:**
+
+* Time complexity : O(n).
+* Space complexity : O(n).