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.

思路：扫两遍，第一遍记录每个char的最后的位置，第二次，看char的最后位置是否比maxend大，如果大就更新，否则如果i == maxend表明，从start到maxend是个 一个valid string，然后start = i + 1 以此类推；

主要是记录最后一个char比较难想，然后更新最后的maxend也比较难点，其余就是two pointer的想法；

class Solution {
    public List<Integer> partitionLabels(String S) {
        List<Integer> list = new ArrayList<Integer>();
        if(S == null || S.length() == 0) {
            return list;
        }
        HashMap<Character, Integer> hashmap = new HashMap<>();
        for(int i = 0; i < S.length(); i++) {
            char c = S.charAt(i);
            // store last position of this char;
            hashmap.put(c, i);
        }
        int start = 0; 
        int maxend = 0;
        for(int i = 0; i < S.length(); i++) {
            char c = S.charAt(i);
            if(hashmap.get(c) > maxend) {
                maxend = hashmap.get(c);
            }
            if(i == maxend) {
                list.add(maxend - start + 1);
                maxend = 0;
                start = i + 1;
            }
        }
        return list;
    }
}