标签：ram hash simple dup memory 了解 int math log
Given an array of integers with possible duplicates, randomly output the index of a given target number.
You can assume that the given target number must exist in the array. Note: The array size can be very large. Solution that uses too much extra space will not pass the judge. Example: int[] nums = new int[] {1,2,3,3,3}; Solution solution = new Solution(nums); // pick(3) should return either index 2, 3, or 4 randomly.
Each index should have equal probability of returning. solution.pick(3); // pick(1) should return 0. Since in the array only nums[0] is equal to 1. solution.pick(1);
Three types of answer:
Map Solution, O(N) memory, O(N) init, O(1) pick.
Like @dettier‘s Reservoir Sampling. O(1) init, O(1) memory, but O(N) to pick.
这道题指明了我们不能用太多的空间，那么省空间的随机方法只有水塘抽样了，LeetCode之前有过两道需要用这种方法的题目Shuffle an Array和Linked List Random Node。那么如果了解了水塘抽样，这道题就不算一道难题了，我们定义两个变量，计数器cnt和返回结果res，我们遍历整个数组，如果数组的值不等于target，直接跳过；如果等于target，计数器加1，然后我们在[0,cnt)范围内随机生成一个数字，如果这个数字是0，我们将res赋值为i即可，参见代码如下：
Reservior Sampling:
public class Solution {
int[] arr;
Random random;
public Solution(int[] nums) {
arr = nums;
random = new Random();
}
public int pick(int target) {
int count = 0;
int index = 0;
for (int i=0; i<arr.length; i++) {
if (arr[i] == target) {
count++;
if (random.nextInt(count) == 0) {
index = i;
}
}
}
return index;
}
}
/**
* Your Solution object will be instantiated and called as such:
* Solution obj = new Solution(nums);
* int param_1 = obj.pick(target);
*/
Map solution: MLE for big case
public class Solution {
public Solution(int[] nums) {
for (int i=0; i<nums.length; i++) {
int num = nums[i];
if (!indexes.containsKey(num))
indexes.put(num, new ArrayList<Integer>());
indexes.get(num).add(i);
}
}
public int pick(int target) {
List<Integer> indexes = this.indexes.get(target);
int i = (int) (Math.random() * indexes.size());
return indexes.get(i);
}
private Map<Integer, List<Integer>> indexes = new HashMap<>();
}
标签：ram hash simple dup memory 了解 int math log
原文地址：http://www.cnblogs.com/apanda009/p/7398103.html
