[Leetcode] Binary Tree Right Side View

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.

For example:
Given the following binary tree,

   1            <---

 /   \

2     3         <---

 \     \

  5     4       <---

 

You should return [1, 3, 4].

层次遍历。

 1 /**

 2  * Definition for binary tree

 3  * struct TreeNode {

 4  *     int val;

 5  *     TreeNode *left;

 6  *     TreeNode *right;

 7  *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}

 8  * };

 9  */

10 class Solution {

11 public:

12     vector<int> rightSideView(TreeNode *root) {

13         vector<int> res;

14         if (root == NULL) return res;

15         queue<TreeNode*> Q;

16         TreeNode *p;

17         Q.push(root);

18         int cnt1 = 1, cnt2;

19         while (!Q.empty()) {

20             cnt2 = 0;

21             for (int i = 0; i < cnt1; ++i) {

22                 p = Q.front();

23                 Q.pop();

24                 if (i == cnt1 - 1) res.push_back(p->val);

25                 if (p->left) {

26                     Q.push(p->left);

27                     ++cnt2;

28                 }

29                 if (p->right) {

30                     Q.push(p->right);

31                     ++cnt2;

32                 }

33             }

34             cnt1 = cnt2;

35         }

36         return res;

37     }

38 };