找出字符串中出现次数最多的字符。例如：bbcaaa，出现次数最多的字符是a。

可以用一个字典来记录每个字符出现的次数，然后遍历字典找出出现次数最多的字符。

代码如下：

def find_most_common_char(s):
    char_count = {}
    for c in s:
        char_count[c] = char_count.get(c, 0) + 1
    max_count = 0
    max_char = None
    for c, count in char_count.items():
        if count > max_count:
            max_count = count
            max_char = c
    return max_char

测试代码：

s = 'bbcaaa'
print(find_most_common_char(s)) # 输出'a'