Linked-List-Random-Node

第90天。

Given a singly linked list, return a random node’s value from the linked list. Each node must have the same probability of being chosen.

Follow up: What if the linked list is extremely large and its length is unknown to you? Could you solve this efficiently without using extra space?

Example:

// Init a singly linked list [1,2,3]. ListNode head = new ListNode(1); head.next = new ListNode(2); head.next.next = new ListNode(3); Solution solution = new Solution(head);

// getRandom() should return either 1, 2, or 3 randomly. Each element should have equal probability of returning. solution.getRandom();

写出了一个朴素的解法，两次扫描：

Solution(ListNode* p) {
    len = 0;
    head = p;
    while(p) {
        len++;
        p = p->next;
    }
    cout << len << endl;
}

/** Returns a random node's value. */
int getRandom() {
    int r = rand() % len;
    cout << r << endl;
    ListNode *p = head;
    while(r-- && p) {
        p = p->next;
    }
    return p->val;
}
int len;
ListNode *head;

然后是利用栈来做的一个解法，即一直递归调用直到链表结尾，这时我们已经遍历了一遍链表就可以知道其长度了，在这时生成随机数，然后在递归调用返回的时候通过这个随机数来选取节点：

int getRandom() {
    temp_len = 0;
    getRandom(head);
    return res;
}
bool getRandom(ListNode *p) {
    if (p == nullptr) {
        rand_n = rand() % temp_len;
        return false;
    }
    temp_len++;
    if(getRandom(p->next)) return true;
    if (rand_n == 0) {
        res = p->val;
        return true;
    }
    rand_n--;
    return false;
}
int temp_len;
int rand_n;
ListNode *head;
int res;

最后是dicuss中的水库抽样法：

int getRandom() {
    int res = head->val;
    ListNode* node = head->next;
    int i = 2;
    while(node){
        int j = rand()%i;
        if(j==0)
            res = node->val;
        i++;
        node = node->next;
    }
    return res;
}

证明可参考：http://blog.csdn.net/so_geili/article/details/52937212