finish 102,103

Author: BaffinLee

101-200/102. Binary Tree Level Order Traversal.md

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+# 102. Binary Tree Level Order Traversal
+
+- Difficulty: Medium.
+- Related Topics: Tree, Breadth-first Search.
+- Similar Questions: Binary Tree Zigzag Level Order Traversal, Binary Tree Level Order Traversal II, Minimum Depth of Binary Tree, Binary Tree Vertical Order Traversal, Average of Levels in Binary Tree.
+
+## Problem
+
+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]
+]
+```
+
+## 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 levelOrder = function(root) {
+  if (!root) return [];
+  return helper([[root]], 0);
+};
+
+var helper = function (res, level) {
+  var now = res[level];
+  var next = [];
+  
+  for (var i = 0; i < now.length; i++) {
+    if (now[i].left) next.push(now[i].left);
+    if (now[i].right) next.push(now[i].right);
+    now[i] = now[i].val;
+  }
+  
+  if (next.length) {
+    res.push(next);
+    helper(res, level + 1);
+  }
+  
+  return res;
+};
+```
+
+**Explain:**
+
+bfs
+
+**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 levelOrder = function(root) {
+  return helper([], root, 0);
+};
+
+var helper = function (res, root, level) {
+  if (root) {
+    if (!res[level]) res[level] = [];
+    res[level].push(root.val);
+    helper(res, root.left, level + 1);
+    helper(res, root.right, level + 1);
+  }
+  return res;
+};
+```
+
+**Explain:**
+
+dfs
+
+**Complexity:**
+
+* Time complexity : O(n).
+* Space complexity : O(n).

101-200/103. Binary Tree Zigzag Level Order Traversal.md

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+# 103. Binary Tree Zigzag Level Order Traversal
+
+- Difficulty: Medium.
+- Related Topics: Stack, Tree, Breadth-first Search.
+- Similar Questions: Binary Tree Level Order Traversal.
+
+## Problem
+
+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]
+]
+```
+
+## Solution
+
+```javascript
+/**
+ * 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 [];
+  return helper([[root]], 0);
+};
+
+var helper = function (res, level) {
+  var now = res[level];
+  var next = [];
+  
+  for (var i = now.length - 1; i >= 0; i--) {
+    if (level % 2) {
+      if (now[i].left) next.push(now[i].left);
+      if (now[i].right) next.push(now[i].right);
+    } else {
+      if (now[i].right) next.push(now[i].right);
+      if (now[i].left) next.push(now[i].left);
+    }
+    
+    now[i] = now[i].val;
+  }
+  
+  if (next.length) {
+    res.push(next);
+    helper(res, level + 1);
+  }
+  
+  return res;
+};
+```
+
+**Explain:**
+
+nope.
+
+**Complexity:**
+
+* Time complexity : O(n).
+* Space complexity : O(n).