348. Largest Odd Number in String

You are given a string num, representing a large integer. Return the largest-valued odd integer (as a string) that is a non-empty substring of num, or an empty string if no odd integer exists.

A substring is a contiguous sequence of characters within a string.

Example:
Input: num = "52"
Output: "5"

Explanation: "5" is the only odd number.

Example 2:
Input: num = "4206"
Output: ""

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