163. Missing Ranges

//非常恶心的一道题。。 错了好多次
class Solution {
    public List<String> findMissingRanges(int[] nums, int lower, int upper) {
        List<String> list = new ArrayList<>();
        int n = nums.length;
        if(n < 1)
        {
            // from lower to upper range -> inclusive are not covered
            String str = toStr(lower, upper);
            if(str != null) list.add(str);
            return list;
        }
        if(lower < nums[0])
        {
            String str = toStr(lower, nums[0] - 1);
            if(str != null) list.add(str);
        }
        for(int i = 0; i < n - 1; i++)
        {
            int a = nums[i], b = nums[i + 1];
            if(a == b || a + 1 >= b) continue;
            String str = toStr(a + 1, b - 1);
            if(str != null) list.add(str);
        }
        if(nums[n - 1] < upper)
        {
            String str = toStr(nums[n - 1] + 1, upper);
            if(str != null) list.add(str);
        }
        return list;
    }

    String toStr(int a, int b)
    {
        if(a > b) return null;
        if(a == b) return "" + a;
        return "" + a + "->" + b;
    }
}