分块入门——优雅的暴力

分块可以干什么？？？
区间修改，区间求值，还可以求一些线段树不容易达成的目标，比方说是区间大于K的数有多少个。

代码

#include <bits/stdc++.h>

using namespace std;

typedef long long ll;

const int maxn = 1e6 + 10;

int n, m, block, belong[maxn], _left[maxn], _right[maxn], num, a[maxn], inc[maxn] = {0};

ll sum[maxn];

void build()
{
    block = sqrt(n);
    num = n / block;
    if(n % block) num++;
    for(int i = 1; i <= num; i++) {
        _left[i] = (i - 1) * block + 1;
        _right[i] = i * block;
    }
    _right[num] = n;
    for(int i = 1; i <= num; i++) {
        for(int j = _left[i]; j <= _right[i]; j++) {
            belong[j] = i;
            sum[i] += a[j];
        }
    }
}
ll ask(int l, int r) {
    ll ret = 0;
    if(belong[l] == belong[r]) {
        for(int i = l; i <= r; i++) {
            ret += a[i];
        }
    }
    else {
        for(int i = l; i <= _right[belong[l]]; i++) {
            ret += a[i] + inc[belong[i]];
        }
        for(int i = belong[l] + 1; i < belong[r]; i++) {
            ret += sum[i] + inc[i] * block;
        }
        for(int i = _left[belong[r]]; i <= r; i++) {
            ret += a[i] + inc[belong[i]];
        }
    }
    return ret;
}
void update(int l, int r, int d) {
    if(belong[l] == belong[r]) {
        for(int i = l; i <= r; i++) {
            a[i]+=d;
            sum[belong[l]]  += d;
        }
    }
    else {
        for(int i = l; i <= _right[belong[l]]; i++) {
            a[i] += d;
            sum[belong[l]]+=d;
        }
        for(int i = belong[l] + 1; i < belong[r]; i++) {
            inc[i] += d;
        }
        for(int i = _left[belong[r]]; i <= r; i++) {
            a[i] += d;
            sum[belong[r]]+=d;
        }
    }
}
int main()
{
    // freopen("i.txt", "r", stdin);
    scanf("%d%d", &n, &m);
    for(int i = 1; i <= n; i++) scanf("%d", &a[i]);
    build();
    while(m--) {
        int op;
        scanf("%d", &op);
        if(op == 2) {
            int l, r;
            scanf("%d%d", &l, &r);
            printf("%lld\n", ask(l,r));
        }
        else {
            int l, r, d;
            scanf("%d%d%d", &l, &r, &d);
            update(l, r, d);
        }
    }
    return 0;
}

/*
5 5
1 5 4 2 3
2 2 4
1 2 3 2
2 3 4
1 1 5 1
2 1 4
*/