①求差值法 ②遍历

static int findMissingNumber(int start, int end, List<Integer> list) {

       int sum = (start + end)*(end - start + 1)/2;

       for(Integer n : list) {

              sum -= n;

        }

       return sum;

    }

static int findMissingNumber(int start, int end, List<Integer> list) {

       int found = 0;

       for(int i = start; i <= end; i++) {

              if(!list.contains(i)) {

                     found = i;

                     break;

              }

       }

       return found;

    }