## [Leetcode]382. Linked List Random Node(C++)

### 题目描述

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.

### 例子

#### 例子 1

Input: `["Solution", "getRandom", "getRandom", "getRandom", "getRandom", "getRandom"]` `[[[1, 2, 3]], [], [], [], [], []]` Output: `[null, 1, 3, 2, 2, 3]` Explanation: `Solution solution = new Solution([1, 2, 3]);` `solution.getRandom(); // return 1` `solution.getRandom(); // return 3` `solution.getRandom(); // return 2` `solution.getRandom(); // return 2` `solution.getRandom(); // return 3` `// getRandom() should return either 1, 2, or 3 randomly. Each element should have equal probability of returning.`

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

### Constraints

• The number of nodes in the linked list will be in the range `[1, 10^4]`.
• `-10^4 <= Node.val <= 10^4`
• At most `10^4` calls will be made to `getRandom`.

### 解题思路

``````class Solution {
public:
Note that the head is guaranteed to be not null, so it contains at least
one node. */
length_ = 0;
while (curr) {
length_++;
curr = curr->next;
}
}

/** Returns a random node's value. */
int getRandom() {
int step = rand() % length_;
while (step-- != 0) {
ret = ret->next;
}
return ret->val;
}

private:
• 时间复杂度: `O(n)`
• 空间复杂度: `O(n)`