#129 Rehashing
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题目描述:
The size of the hash table is not determinate at the very beginning. If the total size of keys is too large (e.g. size >= capacity / 10), we should double the size of the hash table and rehash every keys. Say you have a hash table looks like below:
size=3
, capacity=4
[null, 21, 14, null] ↓ ↓ 9 null ↓ null
The hash function is:
int hashcode(int key, int capacity) { return key % capacity;}
here we have three numbers, 9, 14 and 21, where 21 and 9 share the same position as they all have the same hashcode 1 (21 % 4 = 9 % 4 = 1). We store them in the hash table by linked list.
rehashing this hash table, double the capacity, you will get:
size=3
, capacity=8
index: 0 1 2 3 4 5 6 7hash : [null, 9, null, null, null, 21, 14, null]
Given the original hash table, return the new hash table after rehashing .
Notice
For negative integer in hash table, the position can be calculated as follow:
- C++/Java: if you directly calculate -4 % 3 you will get -1. You can use function: a % b = (a % b + b) % b to make it is a non negative integer.
- Python: you can directly use -1 % 3, you will get 2 automatically.
Given [null, 21->9->null, 14->null, null]
,
return[null, 9->null, null, null, null, 21->null, 14->null, null]
这题需要注意的是每个位置上可能是个linked list,所以从旧位置取出node时,还是需要保持剩下的linked list在旧位置上。把node放进新位置时,需要把它放在新位置的linked list的尾巴上。
Mycode(AC = 23ms):
/** * Definition of ListNode * class ListNode { * public: * int val; * ListNode *next; * ListNode(int val) { * this->val = val; * this->next = NULL; * } * } */class Solution {public: /** * @param hashTable: A list of The first node of linked list * @return: A list of The first node of linked list which have twice size */ vector<ListNode*> rehashing(vector<ListNode*> hashTable) { // write your code here vector<ListNode*> rehashTable(hashTable.size() * 2, NULL); int new_size = hashTable.size() * 2; for (int i = 0; i < hashTable.size(); i++) { if (hashTable[i] != NULL) { // take out the head node of hashTable[i] ListNode *head = hashTable[i], *next = head->next; hashTable[i] = next; head->next = NULL; // compute the new pos, and put the node to the new pos // or end of linked list at new pos int pos = (head->val % new_size + new_size) % new_size; if (rehashTable[pos] == NULL) { rehashTable[pos] = head; } else { ListNode *node = rehashTable[pos]; while (node->next) { node = node->next; } node->next = head; } i--; } } return rehashTable; }};
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