hihocoder1710 等差子数列

思路：

将数列合并之后使用线段树。边界条件容易写错。

实现：

 #include <bits/stdc++.h>
 using namespace std;
 const int MAXN = ;
 const int INF = 0x3f3f3f3f;

 int a[MAXN], tree[MAXN * ], n, m, sum[MAXN];
 vector<int> v(, );

 void build(int num, int l, int r)
 {
     if (l == r) { tree[num] = v[l]; return; }
     int m = l + r >> ;
     build(num * , l, m);
     build(num *  + , m + , r);
     tree[num] = max(tree[num * ], tree[num *  + ]);
 }

 int query(int num, int l, int r, int x, int y)
 {
     if (x <= l && y >= r) return tree[num];
     int ans = -INF, m = l + r >> ;
     if (x <= m) ans = max(ans, query(num * , l, m, x, y));
     if (y >= m + ) ans = max(ans, query(num *  + , m + , r, x, y));
     return ans;
 }

 int main()
 {
     scanf("%d %d", &n, &m);
     for (int i = ; i < n; i++) scanf("%d", &a[i]);
     a[n] = INF;
     int cnt = ;
     for (int i = ; i < n; i++)
     {
         if (a[i] - a[i - ] != a[i + ] - a[i]) { v.push_back(cnt - ); cnt = ; }
         else cnt++;
     }
     int tot = v.size() - ;
     build(, , tot);
     for (int i = ; i <= tot; i++) sum[i] = sum[i - ] + v[i];
     int l, r;
     for (int i = ; i < m; i++)
     {
         scanf("%d %d", &l, &r);
         if (n == ) { puts(""); continue; }
         int ls = lower_bound(sum + , sum + tot + , l == n ? l -  : l) - sum;
         int rs = lower_bound(sum + , sum + tot + , r == n ? r -  : r) - sum;
         int ans = -;
         if (ls == rs) ans = r - l + ;
         else
         {
             ans = max(sum[ls] - l + , r - sum[rs - ]);
             if (rs - ls > ) ans = max(ans, query(, , tot, ls + , rs - ) + );
         }
         printf("%d\n", ans);
     }
     return ;
 }