LeetCode 398: Random Pick Index

class Solution {

    Map<Integer, Integer> startIndex;
    Map<Integer, Integer> lengths;
    Random random;
    public Solution(int[] nums) {
        random = new Random();
        startIndex = new HashMap<>();
        lengths = new HashMap<>();
        int length = 1;
        int start = nums[0];
        startIndex.put(nums[0], 0);
        for (int i = 1; i < nums.length; i++) {
            if (start != nums[i]) {
                lengths.put(start, length);
                length = 1;
                start = nums[i];
                startIndex.put(nums[i], i);
            } else{
                length++;
            }
        }
        lengths.put(start, length);
    }
    
    public int pick(int target) {
        if (!startIndex.containsKey(target)) {
            return 0;
        }
        
        return startIndex.get(target) + random.nextInt(lengths.get(target));
    }
}

/**
 * Your Solution object will be instantiated and called as such:
 * Solution obj = new Solution(nums);
 * int param_1 = obj.pick(target);
 */