从上往下打印二叉树

思路：巧妙利用队列先进先出的性质

/*
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> ans;
        if(root==NULL) return ans;
        queue<TreeNode*> q;
        q.push(root);
        while(!q.empty())
        {
            TreeNode* fr = q.front();
            ans.push_back(fr->val);
            if(fr->left)
            {
                q.push(fr->left);
            }
            if(fr->right)
            {
                q.push(fr->right);
            }
            q.pop();
        }
        return ans;
    }
};