思路：
通过维护区间的左右端点来实现快速区间合并。类似的题目有https://leetcode-cn.com/problems/longest-consecutive-sequence/，不过此题更加简单，需要考虑的情况更少。
实现：
1 class Solution 2 { 3 public: 4 int findLatestStep(vector<int>& arr, int m) 5 { 6 int n = arr.size(); 7 vector<int> link(n + 2, -1); 8 int cnt = 0, res = -1; 9 for (int i = 0; i < n; i++) 10 { 11 int x = arr[i], l = x - 1, r = x + 1; 12 int L = x, R = x; 13 if (link[l] != -1) L = link[l]; 14 if (link[r] != -1) R = link[r]; 15 if (l - L + 1 == m) cnt--; 16 if (R - r + 1 == m) cnt--; 17 if (R - L + 1 == m) cnt++; 18 link[L] = R; link[R] = L; 19 if (cnt > 0) res = i + 1; 20 } 21 return res; 22 } 23 };