public List<String> summaryRanges(int[] nums){
    List<String> list = new ArrayList<>();
    int i = 0, j = 1, n = nums.length;
    for(; j < n; j++){
        if(nums[j] > nums[j-1] + 1){
            addRange(nums[i], nums[j-1], list);
            i = j;
        }
    }
    if(j == n)  addRange(nums[i], nums[j-1], list);//condition to make sure nums's len is not 0.
    return list;
}
public void addRange(int i, int j, List<String> list){
    list.add(Integer.toString(i) + (i == j ? "" : "->" + Integer.toString(j)));
}