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 }