Leecode 102. 二叉树的层序遍历 BFS

原题链接：Leecode 102. 二叉树的层序遍历

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
public:
    vector<vector<int>> levelOrder(TreeNode* root) {
        vector<vector<int>> res;
        if(!root) return res;
        queue<TreeNode* > q;
        q.push(root);
        while(!q.empty())
        {
            int num=q.size();
            res.push_back(vector<int>());
            for(int i=0;i<num;i++)
            {
                root=q.front(); q.pop(); 
                res.back().push_back(root->val);
                if(root->left) q.push(root->left);
                if(root->right) q.push(root->right);
            }
        }
        return res;
    }
};