int stable_partition(vector<int>& a, int s, int e) {
    if (s >= e) return s;
    vector<int> b(a.size());
    int pa1, pa2, pb1, pb2;
    pa1 = pb1 = s; pa2 = pb2 = e;
    int start = s, end = e;
    int key = a[s];
    while (pa1 < pa2) {
        while (pa1 < pa2 && a[pa1] < key)
        {
             a[s
 
++]= a[pa1++];
        }

        while (pa1 < pa2 && a[pa2] >= key) {
             a[e--]= a[pa2--] ;
        }
        if (pa1 < pa2) {
            b[pb1++]= a[pa1++];
            b[pb2--]= a[pa2--] ;
        }
    }
    int i;
    for (i = pb2 + 1; i <= end; i++)
        a[s++] = b[i];
    
 
int idx = pa1;
    for (i = start; i < pb1; i++)
        a[s++] = b[i];

    return idx;
}

穩定的快排