Remove k digits from a number string to make it as small as possible.
Example: num = "", k = . Answer: "". Remove , , . The greedy insight: to get a smaller number, we want leftmost digits to be as small as possible. Tricky: after removing digits, the result might have leading zeros like '' which should become ''. Edge case: removing all digits leaves '', not an empty string. The greedy choice applies left to right because leftmost digits have highest positional value.