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;
    }
}

Incorrect Solutions

References

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

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