Return whether the cards can be rearranged

Question

I solved this question on LeetCode.com:

Alice has a hand of cards, given as an array of integers. Now she wants to rearrange the cards into groups so that each group is size W, and consists of W consecutive cards. Return true if and only if she can. For e.g., for hand=[1,2,3,6,2,3,4,7,8] and W=3, the answer should be true.

as:

class Solution {
public:
    bool isNStraightHand(vector<int>& nums, int k) {
        if(nums.size()%k!=0) return false;
        
        map<int, int> _m;
        for(int num: nums) _m[num]++;
        
        while(_m.size()) {
            auto it=_m.begin();
            int count=1;
            int prev=it->first;
            while(count<=k) {
                it->second--;
                if(count>1 && it->first-prev!=1) return false;
                else prev=it->first;
                count++;
                if(it->second==0) {
                    auto backupIt=it;
                    _m.erase(backupIt);     //am I causing UB here?
                }
                it++;
            }
        }
        
        return true;
    }
};

This works, but it doesn't look like a sturdy solution. I am curious to know if I am causing Undefined Behavior (UB) when erasing the element above. Earlier, I just had _m.erase(it);, but that wasn't good either. I think so, since the official website says:

References and iterators to the erased elements are invalidated.

so, when I do a it++ in the following line, isn't that invalid? That part in particular can probably be improved.

Answer 1

The answer is "yes" it is UB. To fix it just rewrite the part as:

if(it->second==0) 
{
    it = _m.erase(it);
}
else
{
     it++;
}