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Author: BaffinLee

101-200/199. Binary Tree Right Side View.md

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+# 199. Binary Tree Right Side View
+
+- Difficulty: Medium.
+- Related Topics: Tree, Depth-first Search, Breadth-first Search.
+- Similar Questions: Populating Next Right Pointers in Each Node, Boundary of Binary Tree.
+
+## Problem
+
+Given a binary tree, imagine yourself standing on the **right** side of it, return the values of the nodes you can see ordered from top to bottom.
+
+**Example:**
+
+```
+Input: [1,2,3,null,5,null,4]
+Output: [1, 3, 4]
+Explanation:
+
+   1            <---
+ /   \
+2     3         <---
+ \     \
+  5     4       <---
+```
+
+## 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 rightSideView = function(root) {
+  var queue = [{ node: root, level: 0 }];
+  var result = [];
+  var now = null;
+  while (now = queue.shift()) {
+    if (!now.node) continue;
+    result[now.level] = now.node.val;
+    queue.push({ node: now.node.left, level: now.level + 1 });
+    queue.push({ node: now.node.right, level: now.level + 1 });
+  }
+  return result;
+};
+```
+
+**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 rightSideView = function(root) {
+  var result = [];
+  helper(root, 0, result);
+  return result;
+};
+
+var helper = function (node, level, result) {
+  if (!node) return;
+  result[level] = node.val;
+  helper(node.left, level + 1, result);
+  helper(node.right, level + 1, result);
+};
+```
+
+**Explain:**
+
+nope.
+
+**Complexity:**
+
+* Time complexity : O(n).
+* Space complexity : O(n).