40亿个有序不同的数的文件中找一个缺失的数

int split(int* a, int* b, int*c, int alen, int bit)
{
    int biter, citer, i;
    int v=0, re = 0, *t;

    while(bit--){
        v = (1 << bit);
        for(i=biter=citer=0; i < alen; i++) {
            if(a[i] & (1<<bit)) {
                b[biter++] = a[i];
            } else {
                c[citer++] = a[i];
            }
        }
        if(biter <= citer) {
            re += v;
            t = a;
            a = b;
            b = t;
            alen = biter;
        } else {
            t = c;
            c = a;
            a = t;
            alen = citer;
        }
    }
    return re;
}