Remove all characters except the numeric characters from an alphanumeric string.

Example 1:

Input: S = "AA1d23cBB4"
Output: 1234
Explanation: Remove all chacactres 
other than numbers

Example 2:

Input: S = "a1b2c3"
Output: 123
Explanation: Remove all chacactres
other than numbers

Your task:
Your task is to complete the function string() which takes a single string as input and returns the string. You need not take any input or print anything.
 

Expected Time Complexity: O(|S|)
Expected Auxiliary Space: O(1)

Constraints:
1 <= |S| <= 10⁵