Careercup - Facebook面试题 - 5344154741637120

2014-05-02 10:40

题目链接

原题：

Sink Zero in Binary Tree. Swap zero value of a node with non-zero value of one of its descendants
so that no node with value zero could be parent of node with non-zero.

题目：把二叉树中的值为0的节点尽量往下沉，保证所有值为0的节点绝不会有非0的子节点。

解法：我写的算法是O(n^2)的，对于每个值为0的节点，都向下寻找值为非0的节点，如果找不到，就说明没法下沉了；否则继续下沉。

代码：

 // http://www.careercup.com/question?id=5344154741637120
 #include <algorithm>
 #include <iostream>
 #include <sstream>
 #include <string>
 using namespace std;

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

 void constructBinaryTree(TreeNode *&root)
 {
     static string s;
     stringstream sio;
     int val;

     if (cin >> s && s == "#") {
         root = nullptr;
     } else {
         sio << s;
         if (sio >> val) {
             root = new TreeNode(val);
             constructBinaryTree(root->left);
             constructBinaryTree(root->right);
         } else {
             root = nullptr;
         }
     }
 }

 void preorderTraversal(TreeNode *root)
 {
     if (root == nullptr) {
         cout << "# ";
     } else {
         cout << root->val << ' ';
         preorderTraversal(root->left);
         preorderTraversal(root->right);
     }
 }

 class Solution {
 public:
     void sinkZero(TreeNode *root) {
         if (root == nullptr) {
             return;
         }

         if (root->val == ) {
             TreeNode *ptr = findNonZeroNode(root);

             if (ptr != nullptr) {
                 swap(root->val, ptr->val);
             } else {
                 // all zero, no need to go down any more.
                 return;
             }
         }
         sinkZero(root->left);
         sinkZero(root->right);
     };
 private:
     TreeNode *findNonZeroNode(TreeNode *root) {
         if (root == nullptr) {
             return root;
         } else if (root->val != ) {
             return root;
         } else {
             TreeNode *ptr = findNonZeroNode(root->left);
             if (ptr != nullptr) {
                 return ptr;
             } else {
                 return findNonZeroNode(root->right);
             }
         }
     };
 };

 int main()
 {
     TreeNode *root;
     Solution sol;

     while (true) {
         constructBinaryTree(root);
          if (root == nullptr) {
             break;
         }
         sol.sinkZero(root);
         preorderTraversal(root);
         cout << endl;
     }

     return ;
 }