Given a non-negative number N in the form of a string.Find the largest number that can be formed by swapping two characters at most once.

Example 1:
Input:
N=768
Output:
867
Explanation:
Swapping the 1st and 3rd
characters(7 and 8 respectively),
gives the largest possible number.
Example 2:
Input:
N=333
Output:
333
Explanation:
Performing any swaps gives the
same result i.e 333.

Your Task:
You don't need to read input or print anything. Your task is to complete the function LargestSwap() which takes the string N as input parameter and returns the maximum possible string possible with at most one swap.
Expected Time Complexity:O(|N|)
Expected Auxillary Space:O(1)
Constraints:
1<=|N|<=10^{5}

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