Intuition

The most frequent task dictates the skeleton: place its copies with n gaps between them, forming (maxFreq - 1) frames of size (n + 1), then fill the last frame with the tasks that share the max frequency. If there are enough other tasks to fill all idle slots, the answer is just the total number of tasks.

Approach

Count frequencies; find maxFreq and how many tasks have it (maxCount).
Compute (maxFreq - 1) * (n + 1) + maxCount.
The answer is the max of that and the total task count (when no idling is needed).

Complexity

Time — O(N): a single pass to count frequencies.
Space — O(1): at most 26 counts.

Common Pitfalls

Take the max with the total task count — with small n there may be no idle time at all.

Full Solution — all languages

class Solution:
    def leastInterval(self, s, k):
        cnt={}
        for ch in s: cnt[ch]=cnt.get(ch,0)+1
        maxf=max(cnt.values())
        maxc=sum(1 for f in cnt.values() if f==maxf)
        return max(len(s), (maxf-1)*(k+1)+maxc)

import java.util.*;

class Solution {
    public int leastInterval(String s, int k) {
        Map<Character,Integer> cnt=new HashMap<>(); for(char ch:s.toCharArray())cnt.merge(ch,1,Integer::sum); int maxf=0; for(int f:cnt.values())maxf=Math.max(maxf,f); int maxc=0; for(int f:cnt.values())if(f==maxf)maxc++; return Math.max(s.length(),(maxf-1)*(k+1)+maxc);
    }
}

#include <bits/stdc++.h>
using namespace std;

class Solution {
public:
    int leastInterval(string s, int k) {
        map<char,int> cnt; for(char ch:s)cnt[ch]++; int maxf=0; for(auto& kv:cnt)maxf=max(maxf,kv.second); int maxc=0; for(auto& kv:cnt)if(kv.second==maxf)maxc++; return max((int)s.size(),(maxf-1)*(k+1)+maxc);
    }
};

/**
 * @param {string} s
 * @param {number} k
 * @return {number}
 */
var leastInterval = function(s, k) {
    const cnt={}; for(const ch of s)cnt[ch]=(cnt[ch]||0)+1; let maxf=0; for(const kk in cnt)maxf=Math.max(maxf,cnt[kk]); let maxc=0; for(const kk in cnt)if(cnt[kk]===maxf)maxc++; return Math.max(s.length,(maxf-1)*(k+1)+maxc);
};

Task Scheduler

Example 1

Example 2

Constraints