uva 100 The 3n + 1 problem (RMQ)

uva.onlinejudge.org/index.php?option=com_onlinejudge&Itemid=8&page=show_problem&problem=36

　　预处理，RMQ求区间最大值。

代码如下：

#include <cstdio>
#include <iostream>
#include <cstring>
#include <algorithm>
#include <cmath>

using namespace std;

typedef long long LL;
const int N = 4444444;
const int M = 22;
int pre[N], RMQ[M][N >> 2];
int dfs(LL n) {
    if (n <= 0) cout << n << endl;
    if (n < N && pre[n]) return pre[n];
    int tmp;
    if (n & 1) tmp = dfs(n * 3 + 1) + 1;
    else tmp = dfs(n >> 1) + 1;
    if (n < N) pre[n] = tmp;
    return tmp;
}

void PRE() {
    pre[1] = 1;
    for (int i = 2, end = N >> 2; i < end; i++) if (pre[i] == 0) dfs(i);
//    for (int i = 1; i < 20; i++) cout << i << ' ' << pre[i] << endl;
    // prepare RMQ
    for (int i = 0, end = N >> 2; i < end; i++) RMQ[0][i] = pre[i];
    for (int i = 1; i < M; i++) {
        for (int j = 0, end = (N >> 2) - (1 << i); j <= end; j++) {
            RMQ[i][j] = max(RMQ[i - 1][j], RMQ[i - 1][j + (1 << i - 1)]);
        }
    }
}

int query(int l, int r) {
    if (l > r) swap(l, r);
    int ep = (int) log2((double) r - l + 1);
    return max(RMQ[ep][l], RMQ[ep][r - (1 << ep) + 1]);
}

int main() {
    PRE();
    int l, r;
    while (cin >> l >> r) cout << l << ' ' << r << ' ' << query(l, r) << endl;
    return 0;
}

——written by Lyon