https://app.codility.com/demo/results/trainingM5ZZT2-YBK/

FrogRiverOne.java

class Solution {
    public int solution(int X, int[] A) {
        // write your code in Java SE 8
        boolean[] checkArray = new boolean[X];
        
        for(int i=0; i<A.length; i++) {
            boolean is_leave = checkArray[A[i] - 1];
            if (is_leave == false) {
                checkArray[A[i]- 1] = true;
                X -= 1;
                if (X == 0) {
                    return i;
                }
            }
        }
        
        return -1;
    }
}