permutaciones.py

def lexico( w ):
    
    x = y = -1

    # Paso 1
    for i in xrange(len(w)-2, -1, -1):
        if ord(w[i]) < ord(w[i+1]):
            x = i
            break

    if x == -1:
        return None

    # Paso 2
    for i in xrange(len(w)-1, x-1, -1):
        if ord(w[x]) < ord(w[i]):
            y = i
            break

    if y == -1:
        return None

    # Paso 3
    w[x], w[y] = w[y], w[x]

    # Paso 4
    p1 = w[:x+1]
    p2 = w[x+1:]
    p2.reverse()
    p1 += p2
    
    # Retornar nueva permutacion
    return p1