123 Number Flip in Python

Suppose we have an integer n, where only digits 1, 2, and 3 are present. We can flip one digit to a 3. The goal is to find the maximum number we can make.

So, if the input is like 11332, then the output will be 31332.

Approach

To solve this, we will follow these steps ?

• Convert the number into a list of digits

• Iterate through each digit from left to right

• If we find the first digit that is not '3', change it to '3'

• Return the modified number

• If all digits are already '3', return the original number

Example

Let us see the following implementation to get better understanding ?

class Solution:
    def solve(self, n):
        digits = list(str(n))
        for x in range(len(digits)):
            if digits[x] != '3':
                digits[x] = '3'
                return int(''.join(digits))
        return n

ob = Solution()
print(ob.solve(11332))

31332

How It Works

The algorithm works by finding the leftmost digit that is not '3' and changing it to '3'. This maximizes the number because:

• We want to change the most significant digit possible

• Changing a digit to '3' (the highest allowed digit) increases the value

• We scan from left to right to find the first opportunity

Alternative Implementation

Here's a more concise version using string operations ?

def maximize_number(n):
    s = str(n)
    for i, digit in enumerate(s):
        if digit != '3':
            return int(s[:i] + '3' + s[i+1:])
    return n

# Test the function
print(maximize_number(11332))
print(maximize_number(3333))
print(maximize_number(1221))

31332
3333
3221

Key Points

• The algorithm has O(n) time complexity where n is the number of digits

• Space complexity is O(n) for storing the digit list

• If all digits are already '3', no change is needed

• We always change the leftmost non-'3' digit for maximum impact

Conclusion

This greedy approach efficiently finds the maximum number by changing the first non-'3' digit from left to right. The key insight is that changing the leftmost digit has the greatest impact on the final value.