[算法总结]three-way partition

procedure three-way-partition(A : array of values, mid : value):
    i ← 0
    j ← 0
    n ← size of A - 1

    while j ≤ n:
        if A[j] < mid:
            swap A[i] and A[j]
            i ← i + 1
            j ← j + 1
        else if A[j] > mid:
            swap A[j] and A[n]
            n ← n - 1
        else:
            j ← j + 1