Missing Ranges 解答

Question

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

For example, given [0, 1, 3, 50, 75], lower = 0 and upper = 99, return ["2", "4->49", "51->74", "76->99"].

Answer

遍历数组，如果arr[i] > arr[i-1] + 1则说明arr[i]与arr[i-1]中有missing range。注意考虑开头和结尾。

 public class Solution {
     public List<String> findMissingRanges(int[] nums, int lower, int upper) {
         List<String> result = new ArrayList<>();
         if (nums == null || nums.length == 0) {
             addRange(result, lower, upper);
             return result;
         }
         int len = nums.length;
         // process head
         if (nums[0] > lower) {
             addRange(result, lower, nums[0] - 1);
         }
         for (int i = 1; i < len; i++) {
             if (nums[i] > nums[i - 1] + 1) {
                 addRange(result, nums[i - 1] + 1, nums[i] - 1);
             }
         }
         // process tail
         if (nums[len - 1] < upper) {
             addRange(result, nums[len - 1] + 1, upper);
         }
         return result;
     }

     private void addRange(List<String> result, int low, int high) {
         StringBuilder sb = new StringBuilder();
         sb.append(low);
         if (high > low) {
             sb.append("->");
             sb.append(high);
         }
         result.add(sb.toString());
     }
 }