map本身有序，存入小写英文单词，希望从小到大再从大到小输出，有误

Question

eg:
"aaaabbbbbcccc"
存入map本身有序变成
a=4
b=5
c=4
希望从前到后
a b c
再从后到前
c b a
遍历过的数字就去掉，不复用，目前代码如下。希望大佬能帮忙看看是哪有问题，谢谢！
这里附上leetcode链接
https://leetcode.com/contest/biweekly-contest-21/problems/increasing-decreasing-string/

报错图片：

直接把下面的代码放到链接里打开即可调试
class Solution {
public:
string sortString(string s) {
if(s.empty()) return "";
map<char,int> mp;
for(int i=0;i<s.size();++i)
mp[s[i]]++;
for(auto &it:mp)
{
cout<<it.first<<" "<<it.second<<endl;
}
string ans="";
while(!mp.empty())
{
for(auto &it:mp)
{
ans+=it.first;
it.second--;
if(it.second==0) mp.erase(it.first);
}
for(auto &it:mp)
{
cout<<it.first<<" "<<it.second<<endl;
}
for(map<char,int>::reverse_iterator it=mp.rbegin();it!=mp.rend();++it)
{
cout<<"**"<<(*it).first<<endl;
ans+=it->first;
it->second--;
if(it->second==0) mp.erase(it->first);
}
cout<<ans<<endl;
for(auto &it:mp)
{
cout<<it.first<<" "<<it.second<<endl;
}
}
return ans;
}
};

Answer 1

感觉你没有理解题目的意思：
问题一：

mp.erase(it->first)
在循环迭代器的时候erase可能会导致迭代器失效

问题二：

            for(auto &it:mp)//这是构造升序字符串
            {
                ans+=it.first;
                it.second--;
                if(it.second==0) mp.erase(it.first);
            }
            for(auto &it:mp)
            {
                cout<<it.first<<" "<<it.second<<endl;
            }
            for(map<char,int>::reverse_iterator it=mp.rbegin();it!=mp.rend();++it)//这是构造降序字符串，但是这次这个循环一定会执行吗，考虑这种字符串：aaabbbccc ，
            {
                cout<<"**"<<(*it).first<<endl;
                ans+=it->first;
                it->second--;
                if(it->second==0) mp.erase(it->first);
            }
            cout<<ans<<endl;
            for(auto &it:mp)
            {
                cout<<it.first<<" "<<it.second<<endl;
            }

附上我的AC代码，供参考：

class Solution {
public:
    string sortString(string s) {
        map<char, int> table;
        for(auto c : s){
            table[c] = table[c] + 1;
        }
        string res;
        bool up = 1;
        while(table.size() != 0){
            bool finish = true; //为了加速退出
            if(up){
                map<char, int>::iterator begin = table.begin();
                map<char, int>::iterator end = table.end();
                up = 0;
                for(; begin != end; begin++){
                    if(begin->second == 0){
                        continue;
                    }
                    finish = false;
                    res += begin->first;
                    begin->second = begin->second - 1;
                }
            }else{
                map<char, int>::reverse_iterator begin = table.rbegin();
                map<char, int>::reverse_iterator end = table.rend();
                up = 1;
                for(; begin != end; begin++){
                    if(begin->second == 0){
                        continue;
                    }
                    finish = false;
                    res += begin->first;
                    begin->second = begin->second - 1;
                }
            }
            if(finish)
                break;
        }
        return res;
    }
};

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