summary ranges

public List<String> (int[] nums) {
    List<String> ans = new ArrayList<>();
    if (nums == null || nums.length == 0)
        return ans;
    int begin = 0;
    for (int i = 1; i <= nums.length; ++i) {
        if (i == nums.length || nums[i] != nums[i-1] + 1) {
            if (i - 1 == begin) {
                ans.add(Integer.toString(nums[begin]));
            } else {
                StringBuilder sb = new StringBuilder();
                sb.append(nums[begin]);
                sb.append("->");
                sb.append(nums[i - 1]);
                ans.add(sb.toString());
            }
            begin = i;
        }
    }
    return ans;
}