1370. Increasing Decreasing String

main.go

// https://leetcode.com/problems/increasing-decreasing-string/

import "strings"

func sortString(s string) string {
    counts := make([]int, 26)
    
    for _, c := range s {
        counts[int(c - 'a')] += 1
    }
    
    var res strings.Builder
    
    for i, j, k := 0, 25, 1; res.Len() < len(s); i, j, k = j, i, -k {
        for p := i; p != j + k; p += k {
            if counts[p] == 0 {
                continue
            }
            res.WriteString(string('a' + p))
            counts[p] -= 1
        }    
    }
    return res.String()
}