102. 二叉樹的層次遍歷

/**
 * 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) {
        
        TreeNode* p=root;
        vector<vector<int>> a;       
        queue<TreeNode*> q;            
        q.push(p);
        
        if(!p)
            return a;
        
        while(!q.empty())
        {
            int size=q.size();
            vector<int> b;  
            while(size--)
            {
                p=q.front();
                if(p->left)
                    q.push(p->left);
                if(p->right)
                    q.push(p->right);
                b.push_back(p->val);
                q.pop();
            }
            a.push_back(b);  
        }
        return a;
    }
};