https://www.acwing.com/problem/content/description/331/

貌似只能够从下往上反推，从上往下不知道走哪个方向好。每次找出这个平台A下落到哪个平台B，再从B的左右端点向上转移。

#include<bits/stdc++.h>
using namespace std;
typedef long long ll;

const int MAXN = 200001;

short st[MAXN * 4 + 5], lazy[MAXN * 4 + 5];

inline void PushDown(int o) {
    if(lazy[o]) {
        lazy[o << 1] = lazy[o];
        st[o << 1] = lazy[o];
        lazy[o << 1 | 1] = lazy[o];
        st[o << 1 | 1] = lazy[o];
        lazy[o] = 0;
    }
}

void Update(int o, int l, int r, int ql, int qr, int v) {
    if(ql <= l && r <= qr) {
        lazy[o] = v;
        st[o] = v;
        return;
    }
    PushDown(o);
    int mid = l + r >> 1;
    if(ql <= mid)
        Update(o << 1, l, mid, ql, qr, v);
    if(qr >= mid + 1)
        Update(o << 1 | 1, mid + 1, r, ql, qr, v);
}

int Query(int o, int l, int r, int x) {
    if(l == r)
        return st[o];
    PushDown(o);
    int mid = l + r >> 1;
    if(x <= mid)
        return Query(o << 1, l, mid, x);
    return Query(o << 1 | 1, mid + 1, r, x);
}

const int P = 100001;

int dpL[30005], dpR[30005];
int L[30005], R[30005];

int main() {
#ifdef Yinku
    freopen("Yinku.in", "r", stdin);
#endif
    int n, s;
    scanf("%d%d", &n, &s);
    s += P;
    L[0] = P, R[0] = P;
    for(int i = 1; i <= n; ++i) {
        scanf("%d%d", &L[i], &R[i]);
        L[i] += P, R[i] += P;
        int ql = Query(1, 1, MAXN, L[i]);
        dpL[i] = min(dpL[ql] + abs(L[i] - L[ql]), dpR[ql] + abs(L[i] - R[ql]));
        int qr = Query(1, 1, MAXN, R[i]);
        dpR[i] = min(dpL[qr] + abs(R[i] - L[qr]), dpR[qr] + abs(R[i] - R[qr]));
        Update(1, 1, MAXN, L[i], R[i], i);
    }
    printf("%d\n", min(dpL[n] + abs(s - L[n]), dpR[n] + abs(s - R[n])));
}

AcWing - 329 - 围栏障碍训练场 = 线段树 + dp