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 = 0): 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 == 0) {
            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 != 0) {
            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 0;
}