Think:
1知識點：線段樹區間更新+lazy標記
2題意：在n個初始化點權為1的點的基礎上進行區間更新，詢問最終狀態n個點的點權之和
3反思：當前題目lazy標記賦值更新

以下為Wrong Answer程式碼——lazy標記更新錯誤

#include <cstdio>
#include <cstring>
#include <algorithm>

using namespace std;

const int N = 101400;

struct Node{
    int sum;
}node[N<<2];

int lazy[N<<2
 
];

void Build(int l, int r, int rt);
void Updata(int rt);
void down(int rt, int l, int r);
void up_v(int L, int R, int v, int l, int r, int rt);

int main(){
    int T, k = 1, n, m, L, R, v;
    scanf("%d", &T);
    while(T--){
        scanf("%d", &n);
        Build(1, n, 1);
        scanf("%d"
 
, &m);
        while(m--){
            scanf("%d %d %d", &L, &R, &v);
            up_v(L, R, v, 1, n, 1);
        }
        printf("Case %d: The total value of the hook is %d.\n", k++, node[1].sum);
    }
    return 0;
}
void Build(int l, int r, int rt){
    lazy[rt] = 0;
    if(l == r){
        node[rt].sum = 1
 
;
        return;
    }
    int mid = (l+r)/2;
    Build(l, mid, rt<<1);
    Build(mid+1, r, rt<<1|1);
    Updata(rt);
}
void Updata(int rt){
    node[rt].sum = node[rt<<1].sum + node[rt<<1|1].sum;
}
void up_v(int L, int R, int v, int l, int r, int rt){
    if(L <= l && r <= R){
        node[rt].sum = v*(r-l+1);
        lazy[rt] = v;
        return;
    }
    down(rt, l, r);
    int mid = (l+r)/2;
    if(L <= mid)
        up_v(L, R, v, l, mid, rt<<1);
    if(R > mid)
        up_v(L, R, v, mid+1, r, rt<<1|1);
    Updata(rt);
}
void down(int rt, int l, int r){
    if(lazy[rt]){
        lazy[rt<<1] += lazy[rt];
        lazy[rt<<1|1] += lazy[rt];
        int mid = (l+r)/2;
        node[rt<<1].sum = lazy[rt]*(mid-l+1);
        node[rt<<1|1].sum = lazy[rt]*(r-mid);
        lazy[rt] = 0;
    }
}

以下為Accepted程式碼

#include <cstdio>
#include <cstring>
#include <algorithm>

using namespace std;

const int N = 101400;

struct Node{
    int sum;
}node[N<<2];

int lazy[N<<2];

void Build(int l, int r, int rt);
void Updata(int rt);
void down(int rt, int l, int r);
void up_v(int L, int R, int v, int l, int r, int rt);

int main(){
    int T, k = 1, n, m, L, R, v;
    scanf("%d", &T);
    while(T--){
        scanf("%d", &n);
        Build(1, n, 1);
        scanf("%d", &m);
        while(m--){
            scanf("%d %d %d", &L, &R, &v);
            up_v(L, R, v, 1, n, 1);
        }
        printf("Case %d: The total value of the hook is %d.\n", k++, node[1].sum);
    }
    return 0;
}
void Build(int l, int r, int rt){
    lazy[rt] = 0;
    if(l == r){
        node[rt].sum = 1;
        return;
    }
    int mid = (l+r)/2;
    Build(l, mid, rt<<1);
    Build(mid+1, r, rt<<1|1);
    Updata(rt);
}
void Updata(int rt){
    node[rt].sum = node[rt<<1].sum + node[rt<<1|1].sum;
}
void up_v(int L, int R, int v, int l, int r, int rt){
    if(L <= l && r <= R){
        node[rt].sum = v*(r-l+1);
        lazy[rt] = v;
        return;
    }
    down(rt, l, r);
    int mid = (l+r)/2;
    if(L <= mid)
        up_v(L, R, v, l, mid, rt<<1);
    if(R > mid)
        up_v(L, R, v, mid+1, r, rt<<1|1);
    Updata(rt);
}
void down(int rt, int l, int r){
    if(lazy[rt]){
        lazy[rt<<1] = lazy[rt];/*lazy標記賦值更新*/
        lazy[rt<<1|1] = lazy[rt];/*lazy標記賦值更新*/
        int mid = (l+r)/2;
        node[rt<<1].sum = lazy[rt]*(mid-l+1);
        node[rt<<1|1].sum = lazy[rt]*(r-mid);
        lazy[rt] = 0;
    }
}