[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。

/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    vector<int> rightSideView(TreeNode *root) {
        vector<int> res;
        if (root == NULL) return res;
        queue<TreeNode*> Q;
        TreeNode *p;
        Q.push(root);
        int cnt1 = 1, cnt2;
        while (!Q.empty()) {
            cnt2 = 0;
            for (int i = 0; i < cnt1; ++i) {
                p = Q.front();
                Q.pop();
                if (i == cnt1 - 1) res.push_back(p->val);
                if (p->left) {
                    Q.push(p->left);
                    ++cnt2;
                }
                if (p->right) {
                    Q.push(p->right);
                    ++cnt2;
                }
            }
            cnt1 = cnt2;
        }
        return res;
    }
};