0763-partition-labels.js

/**
 * @param {string} s
 * @return {number[]}
 */
var partitionLabels = function (s) {
    var lastIndex = {}, // char -> last index in s
        res = [],
        size = 0,
        end = 0;

    for (var i in s) {
        lastIndex[s[i]] = i;
    }

    for (var i in s) {
        var c = s[i];
        size++;
        end = Math.max(end, lastIndex[c]);
        if (i === String(end)) {
            res.push(size);
            size = 0;
        }
    }

    return res;
};

/**
 * https://leetcode.com/problems/partition-labels/
 * Time O(N) | Space(1)
 * @param {string} S
 * @return {number[]}
 */
 var partitionLabels = function(S) {
    const lastSeen = getLast(S);

    return getAns(S, lastSeen);
};

const getLast = (S, lastSeen = []) => {
    for (const index in S) {/* Time O(N) */
        const code = getCode(S[Number(index)]);
    
        lastSeen[code] = Number(index);/* Space O(1) */
    }

    return lastSeen;
};

const getCode = (char ) => char.charCodeAt(0) - 'a'.charCodeAt(0);

const getAns = (S, lastSeen, left = 0, right = 0, labels = []) => {
    for (const index in S) {/* Time O(N) */
        const code = getCode(S[Number(index)]);
        const lastSeenAt = lastSeen[code];
        
        right = Math.max(right, lastSeenAt);
        
        const isEqual = Number(index) === right;
        if (!isEqual) continue;
        
        const placement = (Number(index) - left) + 1;
        
        labels.push(placement);
        left = Number(index) + 1;   
    };
    
    return labels;
}