为什么

for (int k = 0; k < n; ++k) 
    if ((i >> k) & 1) 
        f[k + 1][i] = min (f[k + 1][i], f[j + 1][i - (1 << k)] + w[j + 1][k + 1]);

能 AC

但

for (int k = 0; k < n; ++k) 
    if (~((i >> k) & 1)) 
        f[k + 1][i - (1 << k)] = min (f[k + 1][i | (1 << k)], f[j + 1][i] + w[j + 1][k + 1]);

WA 60