广度优先访问二叉树

思路：
广度优先，使用队列，注意判断当前值是否为空指针

代码

/*
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) {
        // 广度优先算法，使用队列
        std::vector<int> res;
        std::deque<TreeNode*> q;
        q.push_back(root);
        while(!q.empty()){
            auto ele = q.front();
            q.pop_front();

            if(ele==nullptr){
                continue;
            }

            res.push_back(ele->val);
            q.push_back(ele->left);
            q.push_back(ele->right);

        }
        return res;

    }
};