LeetCode 763. Partition Labels (javascript)

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.

Idea:

• Create hash table to track the last index of the letter
• use start and end variable to decide the partition letters
• when end reach to the last index of the letter :
  • store the length of the partition letters (end – start + 1)
  • reset start => end + 1

Solution:

/**
 * @param {string} S
 * @return {number[]}
 */
var partitionLabels = function(S) {
    let last_index = new Map();
    for (let i = 0; i < S.length; i++) {
        last_index.set(S[i],  i);
    }
    let res = [];
    let start = 0;
    let end = 0;
    for (let i = 0; i < S.length; i++) {
        end = Math.max(end, last_index.get(S[i]));
        if (i === end) {
            res.push(end - start + 1);
            start = end + 1;
        }
    }
    return res;
};