Just did a quick look through your code, and I noticed an error. You might get overflow because n_maxw_max = 10¹¹ which doesn't fit in an int. But the error crashing your code is something different.

The following is a very slight accepted fix 51069859 of your code with the following notes:

1. Reversing the 'ans' string is valid only when the string representation of the node is a palindrome string. For example, reversing 'ans' when one of the vertices of the optimal path is 24 will produce 42 (which is wrong answer) as "24" is not a palindrome string. The simple fix is to use a stack, vector, deque, etc. for storing the vertices, and printing the number in reversed order.

2. The total distance of the optimal path exceeds INT_MAX for some test cases. Hence, the data type for storing the optimal distance should be upgraded to 64-bit.

3. The constant used for INF was not large enough for some test cases. Setting such value to 10¹⁴ was sufficient to pass the system tests.

The following is another accepted solution with further update to your code 51072220.