# 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 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<>());
}

}
}

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;
}
}
``````

## Incorrect Solutions

https://www.cnblogs.com/grandyang/p/5875509.html