2019杭電多校 HDU 6601 Keen On Everything But Triangle 線段樹 或 主席樹

題目連結：https://vjudge.net/problem/HDU-6601

題意：區間組成最大的三角形周長是多少

題解：根據斐波那契的性質，在1e9内最多有47個數就一定能組成三角形，是以線段樹維護下區間的前50大的數，然後每次查詢的時候用滾動數組更新一下即可，當然也可以用主席樹寫，先離散化，記錄下每個數的數目，查詢前50大即可

#include <bits/stdc++.h>
using namespace std;
const int N = 1e5 + 10;
typedef long long ll;
struct node {
    int l, r, len;
    int val[52];
}tree[N << 2];
int n, m, a[N];
int q[2][65];
int qlen[2];
int pos;
void pushup(int cur) {
    tree[cur].len = 0;
    int len1 = 1, len2 = 1;
    while(tree[cur].len < 50 && (len1 <= tree[cur << 1].len || len2 <= tree[cur << 1 | 1].len)) {
        if(len2 > tree[cur << 1 | 1].len || (len1 <= tree[cur << 1].len && tree[cur << 1].val[len1] >= tree[cur << 1 | 1].val[len2])) {
            tree[cur].len++;
            tree[cur].val[tree[cur].len] = tree[cur << 1].val[len1];
            len1++;
        } else {
            tree[cur].len++;
            tree[cur].val[tree[cur].len] = tree[cur << 1 | 1].val[len2];
            len2++;
        }
    }
}
void build(int l, int r, int cur) {
    tree[cur].l = l;
    tree[cur].r = r;
    if(l == r) {
        tree[cur].val[1] = a[l];
        tree[cur].len = 1;
        return;
    }
    int mid = (r + l) >> 1;
    build(l, mid, cur << 1);
    build(mid + 1, r, cur << 1 | 1);
    pushup(cur);
}
void query(int pl, int pr, int cur) {
    if(pl <= tree[cur].l && tree[cur].r <= pr) {
        qlen[pos ^ 1] = 0;
        int i = 1, j = 1;
        while(qlen[pos ^ 1] < 50 &&(i <= qlen[pos] || j <= tree[cur].len)) {
            if(j > tree[cur].len || (i <= qlen[pos] && q[pos][i] >= tree[cur].val[j])) {
                qlen[pos ^ 1] ++;
                q[pos ^ 1][qlen[pos ^ 1]] = q[pos][i];
                i++;
            } else {
                qlen[pos ^ 1] ++;
                q[pos ^ 1][qlen[pos ^ 1]] = tree[cur].val[j];
                j++;
            }
        }
        pos = pos ^ 1;
        return;
    }
    if(pl <= tree[cur << 1].r) query(pl, pr, cur << 1);
    if(pr >= tree[cur << 1 | 1].l) query(pl, pr, cur << 1 | 1);
    return;
}
int main() {
    int l, r;
    ll ans, cnt1, cnt2;
    while(~scanf("%d %d", &n, &m)) {
        for(int i = 1; i <= n; i++) {
            scanf("%d", &a[i]);
        }
        build(1, n, 1);
        while(m--) {
            scanf("%d %d", &l, &r);
    
            qlen[0] = qlen[1] = 0;
            pos = 0;
            query(l, r, 1);
            if(qlen[pos] <= 2) {
                printf("-1\n");
                continue;
            }
            ans = 0;
            for(int i = 1; i + 2 <= qlen[pos]; i++) {
                if(q[pos][i] < q[pos][i + 1] + q[pos][i + 2]) {
                    ans = 1LL * q[pos][i] + q[pos][i + 1] + q[pos][i + 2];
                    break;
                }
            }
            
            if(ans == 0) printf("-1\n");
            else printf("%lld\n", ans);
        }
    }
    return 0;
}