3 way quick sort

3-way partitioning:
将数组分为三份：（小于、等于、大于拆分值）

?Let v be partitioning item a[lo].
?Scan i from left to right.
– (a[i] < v): exchange a[lt] with a[i]; increment both lt and i
– (a[i] > v): exchange a[gt] with a[i]; decrement gt
– (a[i] == v): increment i

void threeWayQsort(int a[], int lo, int hi)
{
    if(hi <= lo) 
        return;
    int lt = lo;
    int gt = hi;
    int parVal = a[lo];

    int i = lo;
    while(i <= gt)
    {
        if(a[i] < parVal)
            swap(a[i++],a[lt++]); //lt: start pos of (items = parVal)
        else if(a[i] > parVal)    //gt: end pos of (items = parVal)
            swap(a[i],a[gt--]);
        else
            i++;
    }
    threeWayQsort(a, lo, lt-1);
    threeWayQsort(a, gt+1, hi);
}

Application：
Color Sort: