CareerCup Compute the rearrangement of x that is closest to y but still greater than y

From the set of natural integer numbers 
Let x = 1234 = {1, 2, 3, 4} 
Let y = 2410 = {2, 4, 1, 0} 

Write an algorithm to compute the rearrangement of x that is closest to y but still greater than y. Both x and y have the same number of digits. 

So in the example above, the answer would be { 2, 4, 1, 3 } = 2413 which is greater than y = 2410 and closer than any other arrangements of x. 

And whats the time complexity of this algorithm?

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backtracing. 
1. sort x first. 
2. iterate through x, find a unused digit that is greater than or equal to the target digit in y. 
2.a if we can find a digit that is strictly greater than that in y, we find a solution, add all unused digits in x into the result in ascending order 
2.b if we can only find a digit in x that is equal to the target digit in y, we continue this execution path.