整段修改刚开始还求前后两值的差，卡了一下发现直接改就行了。
题意和题解： 线段树板子区间修改区间查询，由于本题查一整段所以无需查询操作

代码：

//修改区间为同一个值 
#include<cstdio>
#include<cstring>
#include<algorithm>

using namespace std;

const int N = 1e5 + 10;
int T, n, m;

struct Node{
	int l, r;
	int sum, add;
}tr[N << 2];

void pushup(int u) {
	tr[u].sum = tr[u << 1].sum + tr[u << 1 | 1].sum;
}

void pushdown(int u) {
	Node &root = tr[u], &left = tr[u << 1], &right = tr[u << 1 | 1];
	if(root.add) {
		left.add = right.add = root.add;
		int mid = root.l + root.r >> 1;
		left.sum = (mid - root.l + 1) * root.add;
		right.sum = (root.r - (mid + 1) + 1) * root.add;
		root.add = 0;
	}
}

void build(int u, int l, int r) {
	tr[u] = {l, r, 1, 0};
	if(l == r) return ;
	
	int mid = l + r >> 1;
	build(u << 1, l, mid), build(u << 1 | 1, mid + 1, r);
	pushup(u);
}

void modify(int u, int l, int r, int x) {
	if(tr[u].l >= l && tr[u].r <= r) {
		tr[u].sum = (tr[u].r - tr[u].l + 1) * x;
		tr[u].add = x;
		return ;
	}
	
	pushdown(u);
	int mid = tr[u].l + tr[u].r >> 1;
	if(l <= mid) modify(u << 1, l, r, x);
	if(r > mid) modify(u << 1 | 1, l, r, x);
	pushup(u);
}

int main()
{
	scanf("%d", &T);
	for(int g = 1; g <= T; g++) {
		scanf("%d%d", &n, &m);
		build(1, 1, n);
		
		while(m--) {
			int x, y, z;
			scanf("%d%d%d", &x, &y, &z);
			modify(1, x, y, z);
		}
		
		printf("Case %d: The total value of the hook is %d.\n", g, tr[1].sum);
	}	
	return 0;
}