bfs二叉树层序遍历

要注意的是二维vector的操作。

/**
 * 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> > res;
        queue<TreeNode* > q;
        if(root == NULL)
        	//or
        	//return vector<vector<int> > {};
            return res;
        int step = 0;
        q.push(root);
        while(!q.empty()){
            int sz = q.size();
            if(q.size()>0)
            	//先初始化，或者是另外开一个一维vector暂时保存，结束后在push_back进res
                res.push_back(vector<int>{});
            for(int i=0;i<sz;i++){
                TreeNode* tmp = q.front();
                q.pop();
                if(tmp->left != NULL)
                    q.push(tmp->left);
                if(tmp->right != NULL)
                    q.push(tmp->right);
                res[step].push_back(tmp->val);
            }
            step++;
        }
        return res;
    }
};