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solutions/199. Binary Tree Right Side View.md

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+# [199. Binary Tree Right Side View](https://leetcode.com/problems/binary-tree-right-side-view/)
+
+# 思路
+此题属于二叉树层序遍历的应用，关于[二叉树层序遍历之前分析](https://github.com/ShusenTang/LeetCode/blob/master/solutions/102.%20Binary%20Tree%20Level%20Order%20Traversal.md)过这里就不说了。
+
+# C++
+## 写法一
+每次循环只访问一个节点，用last指针指向当前层的最后一个节点。
+``` C++
+class Solution {
+public:
+    vector<int> rightSideView(TreeNode* root) {
+        vector<int>res;
+        if(!root) return res;
+        queue<TreeNode *>q;
+        TreeNode *last = root, *p = NULL;
+        q.push(root);
+        
+        while(!q.empty()){
+            p = q.front(); q.pop();
+            if(p -> left) q.push(p -> left);
+            if(p -> right) q.push(p -> right);
+            
+            if(last == p){
+                res.push_back(last -> val);
+                last = q.back();
+            }
+        }
+        return res;
+    }
+};
+```
+
+## 写法二
+每次循环访问一层节点。
+``` C++
+class Solution {
+public:
+    vector<int> rightSideView(TreeNode* root) {
+        vector<int>res;
+        if(!root) return res;
+        queue<TreeNode *>q;
+        TreeNode *p = NULL;
+        q.push(root);
+        
+        while(!q.empty()){
+            res.push_back(q.back() -> val);
+            
+            for(int i = q.size(); i > 0; i--){
+                p = q.front(); q.pop();
+                if(p -> left) q.push(p -> left);
+                if(p -> right) q.push(p -> right);
+            }
+        }
+        return res;
+    }
+};
+```