bfs

102.binary-tree-level-order-traversal.md

+
+## 题目地址
+https://leetcode.com/problems/binary-tree-level-order-traversal/description/
+
+## 题目描述
+```
+Given a binary tree, return the level order traversal of its nodes' values. (ie, from left to right, level by level).
+
+For example:
+Given binary tree [3,9,20,null,null,15,7],
+    3
+   / \
+  9  20
+    /  \
+   15   7
+return its level order traversal as:
+[
+  [3],
+  [9,20],
+  [15,7]
+]
+```
+
+## 思路
+
+这道题可以借助`队列`实现，首先把root入队，然后入队一个特殊元素Null(来表示每层的结束)。
+
+
+然后就是while(queue.length), 每次处理一个节点，都将其子节点（在这里是left和right）放到队列中。
+
+然后不断的出队， 如果出队的是null，则表式这一层已经结束了，我们就继续push一个null。
+
+
+## 关键点解析
+
+- 队列
+
+- 队列中用Null(一个特殊元素)来划分每层
+
+- 树的基本操作- 遍历 - 层次遍历（BFS）
+
+
+## 代码
+
+```js
+/*
+ * @lc app=leetcode id=102 lang=javascript
+ *
+ * [102] Binary Tree Level Order Traversal
+ *
+ * https://leetcode.com/problems/binary-tree-level-order-traversal/description/
+ *
+ * algorithms
+ * Medium (47.18%)
+ * Total Accepted:    346.4K
+ * Total Submissions: 731.3K
+ * Testcase Example:  '[3,9,20,null,null,15,7]'
+ *
+ * Given a binary tree, return the level order traversal of its nodes' values.
+ * (ie, from left to right, level by level).
+ *
+ *
+ * For example:
+ * Given binary tree [3,9,20,null,null,15,7],
+ *
+ *
+ * ⁠   3
+ * ⁠  / \
+ * ⁠ 9  20
+ * ⁠   /  \
+ * ⁠  15   7
+ *
+ *
+ *
+ * return its level order traversal as:
+ *
+ * [
+ * ⁠ [3],
+ * ⁠ [9,20],
+ * ⁠ [15,7]
+ * ]
+ *
+ *
+ */
+/**
+ * Definition for a binary tree node.
+ * function TreeNode(val) {
+ *     this.val = val;
+ *     this.left = this.right = null;
+ * }
+ */
+/**
+ * @param {TreeNode} root
+ * @return {number[][]}
+ */
+var levelOrder = function(root) {
+  if (!root) return [];
+  const items = []; // 存放所有节点
+  const queue = [root, null]; // null 简化操作
+  let levelNodes = []; // 存放每一层的节点
+
+  while (queue.length > 0) {
+    const t = queue.shift();
+
+    if (t) {
+      levelNodes.push(t.val)
+      if (t.left) {
+        queue.push(t.left);
+      }
+      if (t.right) {
+        queue.push(t.right);
+      }
+    } else { // 一层已经遍历完了
+      items.push(levelNodes);
+      levelNodes = [];
+      if (queue.length > 0) {
+        queue.push(null)
+      }
+    }
+  }
+
+  return items;
+};
+
+```

103.binary-tree-zigzag-level-order-traversal.md

+
+## 题目地址
+https://leetcode.com/problems/binary-tree-zigzag-level-order-traversal/description/
+
+## 题目描述
+和leetcode 102 基本是一样的，思路是完全一样的。
+
+```
+Given a binary tree, return the zigzag level order traversal of its nodes' values. (ie, from left to right, then right to left for the next level and alternate between).
+
+For example:
+Given binary tree [3,9,20,null,null,15,7],
+    3
+   / \
+  9  20
+    /  \
+   15   7
+return its zigzag level order traversal as:
+[
+  [3],
+  [20,9],
+  [15,7]
+]
+```
+
+## 思路
+
+这道题可以借助`队列`实现，首先把root入队，然后入队一个特殊元素Null(来表示每层的结束)。
+
+
+然后就是while(queue.length), 每次处理一个节点，都将其子节点（在这里是left和right）放到队列中。
+
+然后不断的出队， 如果出队的是null，则表式这一层已经结束了，我们就继续push一个null。
+
+
+## 关键点解析
+
+- 队列
+
+- 队列中用Null(一个特殊元素)来划分每层
+
+- 树的基本操作- 遍历 - 层次遍历（BFS）
+
+
+## 代码
+
+```js
+/*
+ * @lc app=leetcode id=103 lang=javascript
+ *
+ * [103] Binary Tree Zigzag Level Order Traversal
+ *
+ * https://leetcode.com/problems/binary-tree-zigzag-level-order-traversal/description/
+ *
+ * algorithms
+ * Medium (40.57%)
+ * Total Accepted:    201.2K
+ * Total Submissions: 493.7K
+ * Testcase Example:  '[3,9,20,null,null,15,7]'
+ *
+ * Given a binary tree, return the zigzag level order traversal of its nodes'
+ * values. (ie, from left to right, then right to left for the next level and
+ * alternate between).
+ * 
+ * 
+ * For example:
+ * Given binary tree [3,9,20,null,null,15,7],
+ * 
+ * ⁠   3
+ * ⁠  / \
+ * ⁠ 9  20
+ * ⁠   /  \
+ * ⁠  15   7
+ * 
+ * 
+ * 
+ * return its zigzag level order traversal as:
+ * 
+ * [
+ * ⁠ [3],
+ * ⁠ [20,9],
+ * ⁠ [15,7]
+ * ]
+ * 
+ * 
+ */
+/**
+ * Definition for a binary tree node.
+ * function TreeNode(val) {
+ *     this.val = val;
+ *     this.left = this.right = null;
+ * }
+ */
+/**
+ * @param {TreeNode} root
+ * @return {number[][]}
+ */
+var zigzagLevelOrder = function(root) {
+  if (!root) return [];   
+  const items = [];
+  let isOdd = true;
+  let levelNodes = [];
+  
+  const queue = [root, null];
+
+
+  while(queue.length > 0) {
+      const t = queue.shift();
+
+      if (t) {
+          levelNodes.push(t.val)
+          if (t.left) {
+            queue.push(t.left)
+          }
+          if (t.right) {
+            queue.push(t.right)
+          }
+      } else {
+        if (!isOdd) {
+          levelNodes = levelNodes.reverse();
+        }
+        items.push(levelNodes)
+        levelNodes = [];
+        isOdd = !isOdd;
+        if (queue.length > 0) {
+            queue.push(null);
+        }
+      }
+  }
+
+  return items
+    
+};
+```
+

README.md

@@ -16,4 +16,7 @@ leetcode题解，记录自己的leecode解题之路。
 - [75.sort-colors.md](https://github.com/azl397985856/leetcode/blob/master/75.sort-colors.md)
 - [86.partition-list](./86.partition-list.md)
 - [92.reverse-linked-list-ii](./92.reverse-linked-list-ii.md)
+- [94.binary-tree-inorder-traversal](./94.binary-tree-inorder-traversal.md)
+- [102.binary-tree-level-order-traversal](./102.binary-tree-level-order-traversal.md)
+- [103.binary-tree-zigzag-level-order-traversal](./103.binary-tree-zigzag-level-order-traversal.md)
 ### 高级难度
\ No newline at end of file