Question Definition
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);
Java Solution
class Solution {
private Map<Integer, List<Integer>> map;
private Random random;
public Solution(int[] nums) {
map = new HashMap<>();
random = new Random();
for(int i = 0; i < nums.length; i++) {
map.putIfAbsent(nums[i], new LinkedList<>());
map.get(nums[i]).add(i);
}
}
public int pick(int target) {
List<Integer> indexs = map.get(target);
return indexs.get(random.nextInt(indexs.size()));
}
}
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