398. Random Pick Index (Medium)
https://leetcode.com/problems/random-pick-index/
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);
Solutions
1.
class Solution {
// use map to store the connection of value and index, since the give nums
// may contains duplicates, use List to hold all the duplicates of same index
Map<Integer, List<Integer>> map = new HashMap<>();
public Solution(int[] nums) {
for (int i = 0; i < nums.length; i++) {
if (!map.containsKey(nums[i])) {
map.put(nums[i], new ArrayList<>());
}
map.get(nums[i]).add(i);
}
}
public int pick(int target) {
List<Integer> ti = map.get(target);
int ridx = new Random().nextInt(ti.size());
return ti.get(ridx);
}
}
2.
class Solution {
// This solution is less efficient
int[] nums;
public Solution(int[] nums) {
this.nums = nums;
}
public int pick(int target) {
int count = 0;
int idx = -1;
for (int i = 0; i < nums.length; i++) {
if (target != nums[i]) {
continue;
}
count += 1;
if (new Random().nextInt(count) == 0) {
idx = i;
}
}
return idx;
}
}