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使用队列 queue, 取第一个是queue.front();
/**
* 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<vector<int>> levelOrder(TreeNode* root) {
vector<vector<int>> result;
if(root == NULL) return result;
queue<TreeNode *> q;
q.push(root);
while(!q.empty()){
int size = q.size();
vector<int> cur;
for(int i = 0; i < size; i++){
TreeNode * t = q.front();
q.pop();
cur.push_back(t->val);
if(t->left){
q.push(t->left);
}
if(t->right){
q.push(t->right);
}
}
result.push_back(cur);
}
return result;
}
};
