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：

 1 // http://www.careercup.com/question?id=5344154741637120

 2 #include <algorithm>

 3 #include <iostream>

 4 #include <sstream>

 5 #include <string>

 6 using namespace std;

 7 

 8 struct TreeNode {

 9     int val;

10     TreeNode *left;

11     TreeNode *right;

12     TreeNode(int _val = 0): val(_val), left(nullptr), right(nullptr) {};

13 };

14 

15 void constructBinaryTree(TreeNode *&root)

16 {

17     static string s;

18     stringstream sio;

19     int val;

20     

21     if (cin >> s && s == "#") {

22         root = nullptr;

23     } else {

24         sio << s;

25         if (sio >> val) {

26             root = new TreeNode(val);

27             constructBinaryTree(root->left);

28             constructBinaryTree(root->right);

29         } else {

30             root = nullptr;

31         }

32     }

33 }

34 

35 void preorderTraversal(TreeNode *root)

36 {

37     if (root == nullptr) {

38         cout << "# ";

39     } else {

40         cout << root->val << ' ';

41         preorderTraversal(root->left);

42         preorderTraversal(root->right);

43     }

44 }

45 

46 class Solution {

47 public:

48     void sinkZero(TreeNode *root) {

49         if (root == nullptr) {

50             return;

51         }

52         

53         if (root->val == 0) {

54             TreeNode *ptr = findNonZeroNode(root);

55             

56             if (ptr != nullptr) {

57                 swap(root->val, ptr->val);

58             } else {

59                 // all zero, no need to go down any more.

60                 return;

61             }

62         }

63         sinkZero(root->left);

64         sinkZero(root->right);

65     };

66 private:

67     TreeNode *findNonZeroNode(TreeNode *root) {

68         if (root == nullptr) {

69             return root;

70         } else if (root->val != 0) {

71             return root;

72         } else {

73             TreeNode *ptr = findNonZeroNode(root->left);

74             if (ptr != nullptr) {

75                 return ptr;

76             } else {

77                 return findNonZeroNode(root->right);

78             }

79         }

80     };

81 };

82 

83 int main()

84 {

85     TreeNode *root;

86     Solution sol;

87     

88     while (true) {

89         constructBinaryTree(root);

90          if (root == nullptr) {

91             break;

92         }

93         sol.sinkZero(root);

94         preorderTraversal(root);

95         cout << endl;

96     }

97     

98     return 0;

99 }