codility - Counting Elements (FrogRiverOne)

건너지 못하는 경우일 때 return -1

    public int solution(int X, int[] A) {
        // write your code in Java SE 8
 
        boolean[] canJump = new boolean[X + 1];
 
        int pre = 0;
        int index = 0;
        for (; index < A.length; index++) {
            canJump[A[index]] = true;
            if (A[index] == pre + 1) {
                pre++;
                while (pre < X) {
                    if (canJump[pre + 1])
                        pre++;
                    else
                        break;
                }
            }
            if (pre == X)
                break;
        }
        if (pre == X)
            return index;
        else
            return -1;
 
    }