太水了，大佬们就绕道吧……
就是m, b数组分别排个序，然后更改对应位置的m[i]和b[i]，就行了。
因为如果m[i]不改为b[i]而是b[i + 1]的话，那么必定要将m[j] (j > i)改为b[i],而这一定比m[i]改为b[i]更劣。
1 #include<cstdio>
2 #include<iostream>
3 #include<cmath>
4 #include<algorithm>
5 #include<cstring>
6 #include<cstdlib>
7 #include<cctype>
8 #include<vector>
9 #include<stack>
10 #include<queue>
11 using namespace std;
12 #define enter puts("")
13 #define space putchar(' ')
14 #define Mem(a) memset(a, 0, sizeof(a))
15 typedef long long ll;
16 typedef double db;
17 const int INF = 0x3f3f3f3f;
18 const db eps = 1e-8;
19 const int maxn = 2.5e4 + 5;;
20 inline ll read()
21 {
22 ll ans = 0;
23 char ch = getchar(), last = ' ';
24 while(!isdigit(ch)) {last = ch; ch = getchar();}
25 while(isdigit(ch)) {ans = ans * 10 + ch - '0'; ch = getchar();}
26 if(last == '-') ans = -ans;
27 return ans;
28 }
29 inline void write(ll x)
30 {
31 if(x < 0) x = -x, putchar('-');
32 if(x >= 10) write(x / 10);
33 putchar(x % 10 + '0');
34 }
35
36 int n, x, y;
37 int m[maxn], b[maxn];
38 ll ans = 0;
39
40 int main()
41 {
42 n = read(); x = read(); y = read();
43 for(int i = 1; i <= n; ++i) m[i] = read(), b[i] = read();
44 sort(m + 1, m + n + 1); sort(b + 1, b + n + 1);
45 for(int i = 1; i <= n; ++i) ans += (m[i] - b[i]) * (m[i] > b[i] ? y : -x);
46 write(ans); enter;
47 return 0;
48 }
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