The power of the string is the maximum length of a non-empty substring that contains only one unique character.

Given a string `s`, return the power of `s`.

Example 1:

    Input: s = "leetcode"
    Output: 2
    Explanation: The substring "ee" is of length 2 with the character 'e' only.

Example 2:

    Input: s = "abbcccddddeeeeedcba"
    Output: 5
    Explanation: The substring "eeeee" is of length 5 with the character 'e' only.

Constraints:

- `1 <= s.length <= 500`
- `s` consists of only lowercase English letters.