Problem - Remove K Digits

Remove k digits from a number string to make it as small as possible.

Example: num = "$1432219$", k = $3$. Answer: "$1219$". Remove $4$, $3$, $2$. 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 '$0032$' which should become '$32$'. Edge case: removing all digits leaves '$0$', not an empty string. The greedy choice applies left to right because leftmost digits have highest positional value.