# 方式一：递归
def trim(s):
    if s[:1] == ' ':
        return trim(s[1:])
    if s[-1:-2:-1] == ' ':
        return trim(s[:-1])
    return s

# 方式二：查找起始位置
def trim(s):
    b = 0
    e = len(s)
    for i in range(e):
        if s[i] != ' ':
            break
        b += 1
    for i in range(e)[::-1]:
        if s[i] != ' ':
            break
        e = i
    return s[b:e]

# 方式三：查找起始位置
def trim(s):
    while s and s[0] == ' ':
        s = s[1:]
    while s and s[-1] == ' ':
        s = s[:-1]
    return s