《Cracking the Coding Interview》——第4章：树和图——题目4

2014-03-19 03:40

题目：给定一棵二叉树，把每一层的节点串成一个链表，最终返回一个链表数组。

解法：前序遍历，遍历的同时向各个链表里添加节点。水平遍历好像还不如前序遍历来得方便。

代码：

 // 4.4 Level order traversal
 #include <cstdio>
 #include <vector>
 using namespace std;

 struct TreeNode {
     int val;
     TreeNode *left;
     TreeNode *right;

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

 struct ListNode {
     int val;
     ListNode *next;

     ListNode(int _val = ): val(_val), next(nullptr) {};
 };

 void consructBSTFromSortedArray(vector<int> &v, int left, int right, TreeNode *&root)
 {
     if (left > right) {
         root = nullptr;
     } else {
         int mid = (left + right + ) / ;
         root = new TreeNode(v[mid]);
         consructBSTFromSortedArray(v, left, mid - , root->left);
         consructBSTFromSortedArray(v, mid + , right, root->right);
     }
 }

 void preorderTraversal(TreeNode *root, vector<ListNode *> &listHeads, vector<ListNode *> &listTails, int depth)
 {
     if (root == nullptr) {
         printf("# ");
     } else {
         while ((int)listHeads.size() < depth) {
             listHeads.push_back(nullptr);
             listTails.push_back(nullptr);
         }

         if (listHeads[depth - ] == nullptr) {
             listHeads[depth - ] = listTails[depth - ] = new ListNode(root->val);
         } else {
             listTails[depth - ]->next = new ListNode(root->val);
             listTails[depth - ] = listTails[depth - ]->next;
         }

         printf("%d ", root->val);
         preorderTraversal(root->left, listHeads, listTails, depth + );
         preorderTraversal(root->right, listHeads, listTails, depth + );
     }
 }

 void clearBinaryTree(TreeNode *&root)
 {
     if (root == nullptr) {
         return;
     } else {
         clearBinaryTree(root->left);
         clearBinaryTree(root->right);
         delete root;
         root = nullptr;
     }
 }

 void clearList(ListNode *&root)
 {
     ListNode *ptr;

     ptr = root;
     while (ptr != nullptr) {
         root = root->next;
         delete ptr;
         ptr = root;
     }
     root = nullptr;
 }

 int main()
 {
     TreeNode *root;
     int i, n;
     vector<int> v;
     vector<ListNode *> listHeads, listTails;
     ListNode *ptr;

     while (scanf("%d", &n) ==  && n > ) {
         for (i = ; i < n; ++i) {
             v.push_back(i + );
         }

         consructBSTFromSortedArray(v, , n - , root);
         preorderTraversal(root, listHeads, listTails, );
         printf("\n");

         for (i = ; i < (int)listHeads.size(); ++i) {
             printf("Level %d:", i + );
             ptr = listHeads[i];
             while (ptr != nullptr) {
                 printf(" %d", ptr->val);
                 ptr = ptr->next;
             }
             printf("\n");
             clearList(listHeads[i]);
         }

         v.clear();
         clearBinaryTree(root);
         listHeads.clear();
         listTails.clear();
     }

     return ;
 }