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]
]

題意：二叉樹的層序遍歷
思路：BFS，應(yīng)該很熟練了

/**
 * 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) {
         queue<TreeNode*> q;//建立一個樹的隊列
         vector<vector<int>> res;
         if (root == NULL) 
             return res;
        q.push(root);
        while(!q.empty())
        {
            int size=q.size();
            vector<int>v;
            while(size--)
            {
                TreeNode* node=q.front();
                q.pop();
                v.push_back(node->val);
                if(node->left!=NULL)
                    q.push(node->left);
                if(node->right!=NULL)
                    q.push(node->right);
            }
            res.push_back(v);
        }
        return res;
     }
 };