Remove all characters except the numeric characters from an alphanumeric string.
Example 1:
Input: S = "AA1d23cBB4"
Output: 1234
Explanation: Remove all characters
other than numbers
Example 2:
Input: S = "a1b2c3"
Output: 123
Explanation: Remove all characters
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| <= 10^{5}

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见题面