763. Partition Labels

A string S of lowercase letters is given. We want to partition this string into as many parts as possible so that each letter appears in at most one part, and return a list of integers representing the size of these parts.

Example 1:
Input: S = “ababcbacadefegdehijhklij”
Output: [9,7,8]
Explanation:
The partition is “ababcbaca”, “defegde”, “hijhklij”.
This is a partition so that each letter appears in at most one part.
A partition like “ababcbacadefegde”, “hijhklij” is incorrect, because it splits S into less parts.
Note:

S will have length in range [1, 500].
S will consist of lowercase letters (‘a’ to ‘z’) only.

class Solution {
public:
    vector<int> partitionLabels(string S) {
        map<char,int>hash;
        vector<int>ans;
        vector<char>sub;//用来标记当前正在分割子串的字符
        int vis[26],i=0,pre=-1;//pre表示上一段分割子串的最后一个下标位置
        for(i=0;i<S.size();i++) hash[S[i]]++;
        
        for(i=0;i<S.size();i++)
        {//扫描字符串，若遇到hash[cur]==0,检查子串中是否每个字符都为0
            char cur=S[i];
            hash[cur]--;
            if(vis[cur-'a']!=1)//新字符可以进子串
            {
                vis[cur-'a']=true;
                sub.push_back(cur);
            }
            if(hash[cur]==0)//若当前字符词频归零，检查子串中是否每个字符的词频都已归零
            {
            	int j=0;
                for(;j<sub.size();j++)//检查子串中是否每个字符的词频都已归零
                    if(hash[sub[j]]!=0)
                        break;
                if(j==sub.size())//当前子串所有词频都归零
                {
 					ans.push_back(i-pre);
                    sub.clear();//分割完一段子串之后将sub清空
                    pre=i;
                }
            } 
        } 
        return ans;
    }
};