剑指 Offer 32 从上到下打印二叉树

剑指 Offer 32 - I. 从上到下打印二叉树 题目描述

很明显的一道二叉树的层次遍历，用队列实现

C++解法

/* 二叉树镜像
*题目：完成一个函数，输入一个二叉树，输出该二叉树的镜像*/

#include
#include
#include
using namespace std;

struct TreeNode
{
	int val;
	TreeNode* left;
	TreeNode* right;
	TreeNode(int x):val(x),left(nullptr),right(nullptr){}
};

//打印二叉树
static void printTree(TreeNode* node)
{
    if (node != nullptr)
    {
        
        printTree(node->left);
        cout << node->val << " ";
        printTree(node->right);
    }
    
}
void levelOrder(TreeNode* root) {
    if (root == nullptr)
        return ;
    queue<TreeNode*>que;
    que.push(root);
    while (!que.empty())
    {
        int size = que.size();
        auto top = que.front();
        que.pop();
        cout << top->val<<" ";
        //result.push_back(top->val);
        if (top->left)
            que.push(top->left);
        if (top->right)
            que.push(top->right);

    }
   

}
测试代码 
///* 完全二叉树：出来叶子节点，其他节点都有两个子节点
// *          8
// *       /     \
//       6       10 
//      / \      /  \
//      5  7     9   11 
//  */
void test1()
{
    cout << "测试完全二叉树" << endl;
    TreeNode* root = new TreeNode(8);
    root->left = new TreeNode(6);
    root->right= new TreeNode(10);
    root->left->left = new TreeNode(5);
    root->left->right = new TreeNode(7);
    root->right->left = new TreeNode(9);
    root->right->right = new TreeNode(11);
    printTree(root);
    cout << endl;
    cout << "从上打印二叉树为："<<endl;
    levelOrder(root);    
}
/*         8
         /
        6
       /
      5
    /
  9 */
void test2()
{
    cout << "测试二叉树" << endl;
    TreeNode* root = new TreeNode(8);
    root->left = new TreeNode(6);
    root->left->left = new TreeNode(5);
    root->left->left->left = new TreeNode(9);
    printTree(root);
    cout << endl;
    cout << "从上打印二叉树为：" << endl;
    levelOrder(root);
}
void test3()
{
    cout << "测试空节点" << endl;
    printTree(nullptr);
    cout << "从上打印二叉树为：" << endl;
    levelOrder(nullptr);
    
}


int main()
{
    test1();
    cout << endl;
    test2();
    cout << endl;
    test3();
    cout << endl;
    
    return 0;
}

结果

剑指 Offer 32 - II. 从上到下打印二叉树 II 题目解析

与上题的区别就在于，要记录一下，每层的节点数，以及队列中存在的节点数。

代码

class Solution {
public:
    vector<int>result;
    vector<vector<int>> res;
    vector<vector<int>> levelOrder(TreeNode* root) {
         if(root==nullptr)
            return {};
        queue<TreeNode *>que;
        que.push(root);
        int count=0;
        int last=1;//根节点是1
        while(!que.empty())
        {
            int size=que.size();
            auto top=que.front();
            que.pop();
            last--;
            result.push_back(top->val);
            if(top->left)
            {
                que.push(top->left);
                count++;
            }
               
            if(top->right)
            {
                que.push(top->right);
                count++;
            }
            if(last==0)
            {
                res.push_back(result);
                last=count;
                count=0;
                result.clear();
            }
              

        }
        return res;

    }
};

剑指 Offer 32 - III. 从上到下打印二叉树 III 题目描述

代码

class Solution {
public:
    vector<int>result;
    vector<vector<int>> res;
    vector<vector<int>> levelOrder(TreeNode* root) {
         if(root==nullptr)
            return {};
        queue<TreeNode *>que;
        que.push(root);
        int count=0;
        int last=1;
        int num=1;
        while(!que.empty())
        {
            int size=que.size();
            auto top=que.front();
            que.pop();
            last--;
            result.push_back(top->val);
            if(top->left)
            {
                que.push(top->left);
                count++;
            }
               
            if(top->right)
            {
                que.push(top->right);
                count++;
            }
            if(last==0 )
            {
                if(num%2!=0)
                   res.push_back(result);
                else
                {
                    reverse(result.begin(),result.end());
                    res.push_back(result);
                }
                last=count;
                count=0;
                num++;
                result.clear();
            }
        }
        return res;

    }
};