P111: Modified run-length encoding
修改版游程编码。将P110中的游程编码略加修改，当[N, E]项中N是1时，直接简化为E。按照惯例，还是先写测试用例：
from python99.lists.p111 import encode_modified
def test_encode_modified():
assert encode_modified([]) == []
assert encode_modified([1, 2, 2, 2, 3, 4, 5, 5]) == [
1, [3, 2], 3, 4, [2, 5]]
assert encode_modified([1, 1, 2, 2, 2, 3, 3, 4, 4, 4]) == [
[2, 1], [3, 2], [2, 3], [3, 4]]
在P110中已经实现了基本款的「游程编码」，现在祇需要在基本款的结果上再加一层处理，将N为1的[N, E]项转换为E即可。
List comprehension的基本形式为：
[f(x) for e in l]
本例中，f是一个分段函数（piecewise function）：
完整的代码实现：
# Modified run-length encoding.
# Modify the result of problem 1.10 in such a way that if
# an element has no duplicates it is simply copied into the result lists.
# Only elements with duplicates are transferred as [N,E] terms.
from python99.lists.p110 import encode
def encode_modified(l):
return [simplify(e) for e in encode(l)]
def simplify(term):
if term[0] == 1:
return term[1]
else:
return term