LeetCode-102. Binary Tree Level Order Traversal

Given a binary tree, return the level order traversal of its nodes' values. (ie, from left to right, level by level).

For example:
Given binary tree ​​​[3,9,20,null,null,15,7]​​,

3
   / \
  9  20
    /  \
   15   7

 

return its level order traversal as:

[
  [3],
  [9,20],
  [15,7]
]

题解：

class Solution {
public:
    vector<vector<int>> levelOrder(TreeNode* root) {
        queue<TreeNode*> q;
        if (root == NULL) {
            return {};
        }
        q.push(root);
        int cnt = 1;
        vector<int> v;
        vector<vector<int>> res;
        while(q.empty() == false) {
            if (cnt >= 0) {
                cnt--;
                TreeNode *t = q.front();
                q.pop();
                v.push_back(t->val);
                if (t->left != NULL) {
                    q.push(t->left);
                }
                if (t->right != NULL) {
                    q.push(t->right);
                }
            }
            if (cnt == 0) {
                res.push_back(v);
                v.clear();
                cnt = q.size();
            }
        }
        return res;
    }
};