43.不分行从上往下打印二叉树

 

 

 

对二叉树和队列的函数不熟悉，直接看代码分析。

 

/**
 * 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<int> printFromTopToBottom(TreeNode* root) {
        vector<int> res;
        if (!root) return res;
        queue<TreeNode*> q;
        q.push(root);//将根节点都放到队列q里面
        //测试
        cout<<q.front()<<endl;//只返回队头元素
        cout<<q.back()<<endl;//只返回队尾元素
        cout<<q.size()<<endl;//返回队列中元素个数

        while (q.size()) {
            auto t = q.front();
            q.pop();
            cout<< "a=" <<q.size()<<endl;
            res.push_back(t->val);
            if (t->left) q.push(t->left);
            if (t->right) q.push(t->right);
            cout<< "b=" <<q.size()<<endl;
        }

        return res;
        
    }
};

 

 

 

 

注：queue<TreeNode*> q

返回队列头元素：q.front()

返回队列头元素的值q.front()->val