358. Rearrange String k Distance Apart

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Problem

Given a string s and an integer k, rearrange s such that the same characters are at least distance k from each other. If it is not possible to rearrange the string, return an empty string “”.

Example 1:

Input: s = "aabbcc", k = 3
Output: "abcabc"
Explanation: The same letters are at least a distance of 3 from each other.

Example 2:

Input: s = "aaabc", k = 3
Output: ""
Explanation: It is not possible to rearrange the string.

Example 3:

Input: s = "aaadbbcc", k = 2
Output: "abacabcd"
Explanation: The same letters are at least a distance of 2 from each other.

Code

class Solution {
    public String rearrangeString(String s, int k) {
        if(s == null || s.length() == 1) return s;
        
        int[] dict = new int[26];
        int[] valid = new int[26];
        
        for(char c : s.toCharArray()){
            dict[c - 'a']++;
        }
        
        StringBuilder sb = new StringBuilder();
        for(int i = 0; i < s.length(); i++){
            int nextLetter = findNext(dict, valid, i);
            if(nextLetter == -1){
                return "";
            }
            
            sb.append((char)('a' + nextLetter));
            dict[nextLetter]--;
            valid[nextLetter] = i + k;
        }
        
        return sb.toString();
    }
    
    private int findNext(int[] dict, int[] valid, int index){
        int res = -1;
        int max = 0;
        for(int i = 0; i < 26; i++){
            if(max < dict[i] && valid[i] <= index){
                res = i;
                max = dict[i];
            }
        }
        
        return res;
    }
}