# 【題解】AtCoder ABC 177F – I hate Shortest Path Problem

【解題想法】線段樹進階應用，遞增

```#include <iostream>
using namespace std;

int h, w, st[200005 << 2], lazy[200005 << 2], inf = 0x3F3F3F3F;

void push(int x, int l, int mid){
if (!lazy[x]) return;
st[x<<1] = lazy[x];
lazy[x<<1] = lazy[x];
st[x<<1|1] = lazy[x]+(mid-l+1);
lazy[x<<1|1] = lazy[x]+(mid-l+1);
lazy[x] = 0;
}
void update(int x, int l, int r, int ul, int ur, int u){
if (ul <= l && r <= ur){
st[x] = u;
lazy[x] = u;
return;
}
int mid = (l+r) >> 1;
push(x, l, mid);
int del = 0;
if (ul <= mid){
update(x<<1, l, mid, ul, ur, u);
del = mid-max(ul, l)+1;
}
if (mid < ur) update(x<<1|1, mid+1, r, ul, ur, u + del);
st[x] = min(st[x<<1], st[x<<1|1]);
}
int query(int x, int l, int r, int q){
if (q == 0) return inf;
if (l == r) return st[x];
int mid = (l+r) >> 1;
push(x, l, mid);
if (q <= mid) return query(x<<1, l, mid, q);
else return query(x<<1|1, mid+1, r, q);
}

int main() {
cin >> h >> w;
for (int i = 1, a, b; i <= h; i++){
cin >> a >> b;
update(1, 1, w, a, b, query(1, 1, w, a-1)+1);
if (st[1] > w) cout << -1 << "\n";
else cout << st[1]+i << "\n";
}
}

```