從上往下打印二叉樹：

從上往下打印出二叉樹的每個(gè)節(jié)點(diǎn)，同層節(jié)點(diǎn)從左至右打印。

思路：利用bfs思想，構(gòu)建一個(gè)隊(duì)列，每次取出隊(duì)列頭輸出值然后找左右節(jié)點(diǎn)然后加入隊(duì)列，直到隊(duì)列為空。

對(duì)于bfs的理解可參考廣度優(yōu)先算法（bfs）理解+例題

/*
struct TreeNode {
    int val;
    struct TreeNode *left;
    struct TreeNode *right;
    TreeNode(int x) :
            val(x), left(NULL), right(NULL) {
    }
};*/
class Solution {
public:
    vector<int> PrintFromTopToBottom(TreeNode* root) {
        vector<int> s;
        if(root == NULL)return s;
        TreeNode *a;
        a = root;
        
        queue<TreeNode*> q;
        q.push(a);
        while(!q.empty()){
            a = q.front();
            s.push_back(a->val);
            q.pop();
            if(a->left != NULL){
                q.push(a->left);
            }
            if(a->right != NULL){
                q.push(a->right);
            }
            
        }
        return s;
    }
};