线段树

//单点修改、区间查询
int t[maxn]={0};
void build_tree(int node,int start,int end) {
    if(start==end) {
        t[node]=a[start];
        return ;
    }
    int left=2*node+1; //左节点的坐标 
    int right=2*node+2; //右节点的坐标
    int mid=(start+end)/2;
    build_tree(left,start,mid); //递归建树 
    build_tree(right,mid+1,end);
    t[node]=t[left]+t[right];    
}
void update(int node,int start,int end,int index,int val) {
    if(start==end) {
        a[index]=val;
        t[node]=val;
        return ;
    }
    int left=node*2+1;
    int right=node*2+2;
    int mid=(start+end)/2;
    if(index>=start&&index<=mid) {
        update(left,start,mid,index,val);
    }
    else {
        update(right,mid+1,end,index,val);
    }
    t[node]=t[right]+t[left];
}
int query(int node,int start,int end,int L,int R) {
    //不在计算范围之内  
    if(R<start||L>end) return 0;
    //整块在计算范围内 
    else if(L<=start&&R>=end) return t[node];
    //计算到叶节点 
    else if(start==end) return t[node]; 
    
    int mid=(start+end)/2;
    int left=node*2+1;
    int right=node*2+2;
    int sumleft=query(left,start,mid,L,R);
    int sumright=query(right,mid+1,end,L,R);
    return sumleft+sumright;
}