package main

import (
	"bufio"
	"os"
)

func letterSlice(n int) []byte {
	if n <= 0 {
		return []byte{}
	}
	if n > 26 {
		n = 26
	}

	res := make([]byte, n)
	for i := 0; i < n; i++ {
		res[i] = 'a' + byte(i)
	}
	return res
}

// Iterative Heap's algorithm with fast buffered output.
func permuteIterative(nums []byte, w *bufio.Writer) {
	n := len(nums)
	if n == 0 {
		return
	}

	// c is the control array for Heap's algorithm.
	c := make([]int, n)

	// Helper to write one permutation.
	writePerm := func() {
		w.Write(nums)
		w.WriteByte('\n')
	}

	// Print the first permutation (initial order).
	writePerm()

	i := 0
	for i < n {
		if c[i] < i {
			var swapIdx int
			if i%2 == 0 {
				swapIdx = 0
			} else {
				swapIdx = c[i]
			}

			// Swap
			nums[swapIdx], nums[i] = nums[i], nums[swapIdx]

			// Print next permutation
			writePerm()

			c[i]++
			i = 0
		} else {
			c[i] = 0
			i++
		}
	}
}

func main() {
	// Hardcode length 10: a..j
	nums := letterSlice(10)

	// Buffered writer for much faster output than fmt.Printf in a tight loop.
	w := bufio.NewWriter(os.Stdout)
	defer w.Flush()

	permuteIterative(nums, w)
}