FOJ 1608 Huge Mission 线段树

#include<cstdio>
#include<algorithm>
#include<iostream>
#include<cstring>
#include<vector>
#include<stack>
#include<cmath>
#include<queue>
#include<map>
using namespace std;
const int maxn=50005;
int sum[maxn<<2],b[maxn<<2];
void pushup(int rt)
{
    sum[rt]=sum[rt*2]+sum[rt*2+1];
    b[rt]=min(b[rt*2],b[rt*2+1]);
}
void change(int rt,int l,int r,int x,int y,int c)
{
    int m=(l+r)>>1;
    if(l==r)
    {
        sum[rt]=b[rt]=c;
        return;
    }
    if(x<=m&&b[rt*2]<c)change(rt*2,l,m,x,y,c);
    if(y>m&&b[rt*2+1]<c)change(rt*2+1,m+1,r,x,y,c);
    pushup(rt);
}
int main()
{
    int n,m;
    while(~scanf("%d%d",&n,&m))
    {
        int x,y,z;
        memset(sum,0,sizeof(sum));
        memset(b,0,sizeof(b));
        for(int i=0; i<m; i++)
        {
            scanf("%d%d%d",&x,&y,&z);
            change(1,1,n,x+1,y,z);
        }
        printf("%d\n",sum[1]);
    }
    return 0;
}