hihocoder1078 线段树的区间修改

思路：

线段树区间更新。注意这里是把一个区间的所有数全部赋值为一个新的值。

实现：

 #include <bits/stdc++.h>
 using namespace std;
 const int N = ;
 int tree[N << ], lazy[N << ], a[N], n, q;

 void build(int num, int l, int r)
 {
     if (l == r) { tree[num] = a[l]; return; }
     int m = l + r >> ;
     build(num << , l, m);
     build(num <<  | , m + , r);
     tree[num] = tree[num << ] + tree[num <<  | ];
 }

 void pushdown(int num, int cl, int cr)
 {
     if (!lazy[num]) return;
     tree[num << ] = lazy[num] * cl;
     tree[num <<  | ] = lazy[num] * cr;
     lazy[num << ] = lazy[num];
     lazy[num <<  | ] = lazy[num];
     lazy[num] = ;
 }

 void update(int num, int l, int r, int x, int y, int p)
 {
     if (x <= l && y >= r) { tree[num] = (r - l + ) * p; lazy[num] = p; return; }
     int m = l + r >> ;
     pushdown(num, m - l + , r - m);
     if (x <= m) update(num << , l, m, x, y, p);
     if (y >= m + ) update(num <<  | , m + , r, x, y, p);
     tree[num] = 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 m = l + r >> ;
     pushdown(num, m - l + , r - m);
     int ans = ;
     if (x <= m) ans += query(num << , l, m, x, y);
     if (y >= m + ) ans += query(num <<  | , m + , r, x, y);
     return ans;
 }

 int main()
 {
     ios::sync_with_stdio(false);
     cin >> n;
     for (int i = ; i <= n; i++) cin >> a[i];
     build(, , n);
     cin >> q;
     int x, y, t, p;
     for (int i = ; i <= q; i++)
     {
         cin >> t;
         if (t == )
         {
             cin >> x >> y;
             cout << query(, , n, x, y) << endl;
         }
         else
         {
             cin >> x >> y >> p;
             update(, , n, x, y, p);
         }
     }
     return ;
 }