Find the Closest Palindrome

solution.go

// https://leetcode.com/problems/find-the-closest-palindrome/submissions/

import (
	"strconv"
	"strings"
)

const (
	zero = byte('0')
	nine = byte('9')
)

func nearestPalindromic(n string) string {
	m := len(n)
	num, _ := strconv.Atoi(n)

	// case a -> {a-1,a+1}, where a < 9
	if m == 1 {
		return strconv.Itoa(num - 1)
	}

	// case aaaaaaa -> 10000001 where a = 9,
	nines := strings.Repeat("9", m)

	if n == nines {
		return strconv.Itoa(num + 2)
	}

	// case 1000000 -> 999999|1000001
	dec, _ := strconv.Atoi("1" + strings.Repeat("0", m-1))

	if dec == num {
		return strconv.Itoa(num - 1)
	}

	// case 1000001 -> 999999
	if dec+1 == num {
		return strconv.Itoa(num - 2)
	}

	// other cases
	c := m / 2
	part := n[:c]
	v := n[c]
	res := make([]string, 0, 3)

	if m%2 == 0 {
		v = n[c-1]
	}

	for i := v - 1; i <= v+1; i++ {
		candidate := ""

		if i >= zero && i <= nine {
			if m%2 == 1 {
				candidate = part + string(i) + reverse(part)
			} else {
				bpart := []byte(part)
				bpart[c-1] = i
				candidate = string(bpart) + reverse(string(bpart))
			}

			res = append(res, candidate)
		}
	}

	return filter(res, n)
}

func reverse(s string) string {
	r := []rune(s)
	n := len(r)

	for i, j := 0, n-1; i < n/2; i, j = i+1, j-1 {
		r[i], r[j] = r[j], r[i]
	}

	return string(r)
}

func filter(s []string, n string) string {
	result := ""
	min := (1 << 31) - 1
	x, _ := strconv.Atoi(n)

	for _, c := range s {
		if c == n {
			continue
		}

		y, _ := strconv.Atoi(c)
		diff := abs(x - y)

		if min > diff {
			min = diff
			result = c
		} else if result > c {
			result = c
		}
	}

	return result
}

func abs(x int) int {
	if x < 0 {
		return -x
	}

	return x
}