A string S of lowercase English 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 English letters ('a' to 'z') only.

Solution

Time complexity : O(n)
Space complexity : O(1)

class Solution {
public:
    vector<int> partitionLabels(string S) {
        vector<int> last(26, 0);
        for (int i=0; i<S.length(); ++i)
            last[S[i]-'a'] = i;
        
        vector<int> res;
        int start = 0, end = 0;
        for (int i=0; i<S.length(); ++i) {
            end = max(end, last[S[i]-'a']);
            if (i == end) {
                res.push_back(end-start+1);
                start = i+1;
            }
        }
        return res;
    }
};

先記錄下各字母最後的位置，
再利用start、end，算出各區間的長度。