[leetcode]163. Missing Ranges缺失范围

Given a sorted integer array nums, where the range of elements are in the inclusive range [lower, upper], return its missing ranges.

Example:

Input: nums = [0, 1, 3, 50, 75], lower = 0 and upper = 99,
Output: ["2", "4->49", "51->74", "76->99"]

题意：

给定一个区间和一堆数。统计一下在此区间有哪些数没在数组中出现，结果用区间表示。

思路：

这题跟[leetcode]228. Summary Ranges区间统计思路一致

区别是因为规定了  [lower, upper] 要考虑更多的边界

代码：

 class Solution {
     public List<String> findMissingRanges(int[] a, int lower, int upper) {
         List<String> result = new ArrayList<>();

         if(a.length == 0) {
             group(result, lower, upper );
             return result;
         }

         if(a[0] != lower){
             group(result, lower, a[0] - 1);
         }

         for(int i = 1; i < a.length; i++ ){
             if( a[i] != a[i-1] && a[i] != a[i-1] + 1){
                 group(result, a[i-1] + 1, a[i] - 1 );
             }
         }

         if(upper != a[a.length - 1] ){
             group(result, a[a.length - 1] + 1 , upper);
         }
          return result;
     }

     private void group(List<String> list, int start, int end){
         if(start  == end ){
             list.add(start +"");
         }else{
             list.add(start  + "->" + end );
         }
     }
 }