LeetCode 637. Average of Levels in Binary Tree(层序遍历）

Given a non-empty binary tree, return the average value of the nodes on each level in the form of an array.
Example 1:

<b>Input:</b>
    3
   / \
  9  20
    /  \
   15   7
Output: [3, 14.5, 11]
Explanation:
The average value of nodes on level 0 is 3,  on level 1 is 14.5, and on level 2 is 11. Hence return [3, 14.5, 11].

Note:
The range of node’s value is in the range of 32-bit signed integer.
分析

树的层序遍历

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
   vector<double> averageOfLevels(TreeNode* root) {
          vector<double> res;
          double sum=0,cnt=0;
          queue<TreeNode*> q;
          q.push(root);
          TreeNode* last=root;
          TreeNode* tail;
          while(!q.empty()){
               TreeNode* t=q.front();
               q.pop();
               sum+=t->val;
               cnt++;
               if(t->left!=NULL){
                   q.push(t->left);
                   tail=t->left;
               }
               if(t->right!=NULL){
                   q.push(t->right);
                   tail=t->right;
               }
               if(t==last){
                  last=tail;
                  res.push_back(sum/cnt);
                  sum=cnt=0;
               }
          }
        return res;
    }
};