763. Partition Labels - Medium

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.

 

time = O(n), space = O(n)

class Solution {
    public List<Integer> partitionLabels(String S) {
        int[] last = new int[26];
        for (int i = 0; i < S.length(); ++i) {
            last[S.charAt(i) - 'a'] = i; // record the last occurrence of that letter
        }
        
        int start = 0, end = 0;  // the start and end of the current partition
        List<Integer> res = new ArrayList();
        for (int i = 0; i < S.length(); ++i) {
            end = Math.max(end, last[S.charAt(i) - 'a']);
            if (i == end) {
                res.add(i - start + 1);
                start = i + 1;
            }
        }
        return res;
    }
}