给定N(非常大)，任务是打印通过排列N的数字而获得的最大回文数。如果不可能生成回文数，则打印适当的消息。
例子 ：
Input : 313551
Output : 531135
Explanations : 531135 is the largest number
which is a palindrome, 135531, 315513 and other
numbers can also be formed but we need the highest
of all of the palindromes.
Input : 331
Output : 313
Input : 3444
Output : Palindrome cannot be formed
幼稚的方法：幼稚的方法将尝试所有可能的排列，并打印出此类回文中最大的组合。
高效的方法：一种有效的方法是使用贪婪算法。由于数字很大，因此将数字存储在字符串。将给定数字中每个数字的出现次数存储在地图中。检查是否可能形成回文。如果给定数字的数字可以重新排列以形成回文，则可以使用贪婪方法来获取数字。检查是否出现每个数字(9到0)，并将每个可用数字放在前面和后面。最初，前指针将在索引0处，因为最大的数字将首先放置以使数字大。每走一步，将前指针向前移动1个位置。如果该数字出现奇数次，则将一个数字放在偶数个数字的中间和其余部分的前面和后面。保持重复处理(map [digit] / 2)次的位数。在前面和后面放置偶数次的特定数字后，将前指针向前移动一级。放置完成，直到map [digit]为0。char数组将贪婪地完成数字放置后，将具有最大的回文数。
在最坏的情况下，如果数字在每个位置都包含相同的数字，则时间复杂度将为O(10 *(字符串的长度/ 2)) 。
下面是上述想法的实现：
C++
// CPP program to print the largest palindromic
// number by permuting digits of a number
#include
using namespace std;
// function to check if a number can be
// permuted to form a palindrome number
bool possibility(unordered_map m,
int length, string s)
{
// counts the occurrence of number which is odd
int countodd = 0;
for (int i = 0; i < length; i++) {
// if occurrence is odd
if (m[s[i] - '0'] & 1)
countodd++;
// if number exceeds 1
if (countodd > 1)
return false;
}
return true;
}
// function to print the largest palindromic number
// by permuting digits of a number
void largestPalindrome(string s)
{
// string length
int l = s.length();
// map that marks the occurrence of a number
unordered_map m;
for (int i = 0; i < l; i++)
m[s[i] - '0']++;
// check the possibility of a palindromic number
if (possibility(m, l, s) == false) {
cout << "Palindrome cannot be formed";
return;
}
// string array that stores the largest
// permuted palindromic number
char largest[l];
// pointer of front
int front = 0;
// greedily start from 9 to 0 and place the
// greater number in front and odd in the
// middle
for (int i = 9; i >= 0; i--) {
// if the occurrence of number is odd
if (m[i] & 1) {
// place one odd occurring number
// in the middle
largest[l / 2] = char(i + 48);
// decrease the count
m[i]--;
// place the rest of numbers greedily
while (m[i] > 0) {
largest[front] = char(i + 48);
largest[l - front - 1] = char(i + 48);
m[i] -= 2;
front++;
}
}
else {
// if all numbers occur even times,
// then place greedily
while (m[i] > 0) {
// place greedily at front
largest[front] = char(i + 48);
largest[l - front - 1] = char(i + 48);
// 2 numbers are placed, so decrease the count
m[i] -= 2;
// increase placing position
front++;
}
}
}
// print the largest string thus formed
for (int i = 0; i < l; i++)
cout << largest[i];
}
// Driver Code
int main()
{
string s = "313551";
largestPalindrome(s);
return 0;
} Java
// JAVA program to print the
// largest palindromic number
// by permuting digits of a number
import java.util.*;
class GFG{
// Function to check if a number can be
// permuted to form a palindrome number
static boolean possibility(HashMap m,
int length, String s)
{
// counts the occurrence of number
// which is odd
int countodd = 0;
for (int i = 0; i < length; i++)
{
// if occurrence is odd
if (m.get(s.charAt(i) - '0') % 2 == 1)
countodd++;
// if number exceeds 1
if (countodd > 1)
return false;
}
return true;
}
// function to print the largest
// palindromic number by permuting
// digits of a number
static void largestPalindrome(String s)
{
// String length
int l = s.length();
// map that marks the occurrence
// of a number
HashMap m = new HashMap<>();
for (int i = 0; i < l; i++)
if(m.containsKey(s.charAt(i) - '0'))
m.put(s.charAt(i) - '0',
m.get(s.charAt(i) - '0') + 1);
else
m.put(s.charAt(i) - '0', 1);
// check the possibility of a
// palindromic number
if (possibility(m, l, s) == false)
{
System.out.print("Palindrome cannot be formed");
return;
}
// String array that stores
// the largest permuted
// palindromic number
char []largest = new char[l];
// pointer of front
int front = 0;
// greedily start from 9 to 0
// and place the greater number
// in front and odd in the middle
for (int i = 9; i >= 0; i--)
{
// if the occurrence of
// number is odd
if (m.containsKey(i) &&
m.get(i)%2==1)
{
// place one odd occurring
// number in the middle
largest[l / 2] = (char)(i + 48);
// decrease the count
m.put(i, m.get(i)-1);
// place the rest of
// numbers greedily
while (m.get(i) > 0)
{
largest[front] = (char)(i + 48);
largest[l - front - 1] =
(char)(i + 48);
m.put(i, m.get(i) - 2);
front++;
}
}
else
{
// if all numbers occur even
// times, then place greedily
while (m.containsKey(i) &&
m.get(i) > 0)
{
// place greedily at front
largest[front] = (char)(i + 48);
largest[l - front - 1] =
(char)(i + 48);
// 2 numbers are placed,
// so decrease the count
m.put(i, m.get(i) - 2);
// increase placing position
front++;
}
}
}
// print the largest String
// thus formed
for (int i = 0; i < l; i++)
System.out.print(largest[i]);
}
// Driver Code
public static void main(String[] args)
{
String s = "313551";
largestPalindrome(s);
}
}
// This code is contributed by Rajput-Ji Python3
# Python3 program to print the largest palindromic
# number by permuting digits of a number
from collections import defaultdict
# Function to check if a number can be
# permuted to form a palindrome number
def possibility(m, length, s):
# counts the occurrence of
# number which is odd
countodd = 0
for i in range(0, length):
# if occurrence is odd
if m[int(s[i])] & 1:
countodd += 1
# if number exceeds 1
if countodd > 1:
return False
return True
# Function to print the largest palindromic
# number by permuting digits of a number
def largestPalindrome(s):
# string length
l = len(s)
# map that marks the occurrence of a number
m = defaultdict(lambda:0)
for i in range(0, l):
m[int(s[i])] += 1
# check the possibility of a
# palindromic number
if possibility(m, l, s) == False:
print("Palindrome cannot be formed")
return
# string array that stores the largest
# permuted palindromic number
largest = [None] * l
# pointer of front
front = 0
# greedily start from 9 to 0 and place the
# greater number in front and odd in the middle
for i in range(9, -1, -1):
# if the occurrence of number is odd
if m[i] & 1:
# place one odd occurring number
# in the middle
largest[l // 2] = chr(i + 48)
# decrease the count
m[i] -= 1
# place the rest of numbers greedily
while m[i] > 0:
largest[front] = chr(i + 48)
largest[l - front - 1] = chr(i + 48)
m[i] -= 2
front += 1
else:
# if all numbers occur even times,
# then place greedily
while m[i] > 0:
# place greedily at front
largest[front] = chr(i + 48)
largest[l - front - 1] = chr(i + 48)
# 2 numbers are placed,
# so decrease the count
m[i] -= 2
# increase placing position
front += 1
# print the largest string thus formed
for i in range(0, l):
print(largest[i], end = "")
# Driver Code
if __name__ == "__main__":
s = "313551"
largestPalindrome(s)
# This code is contributed by Rituraj JainC#
// C# program to print the largest
// palindromic number by permuting
// digits of a number
using System;
using System.Collections.Generic;
class GFG{
// Function to check if a number can be
// permuted to form a palindrome number
static bool possibility(Dictionary m,
int length, string s)
{
// Counts the occurrence of number
// which is odd
int countodd = 0;
for(int i = 0; i < length; i++)
{
// If occurrence is odd
if ((m[s[i] - '0'] & 1) != 0)
countodd++;
// If number exceeds 1
if (countodd > 1)
return false;
}
return true;
}
// Function to print the largest palindromic
// number by permuting digits of a number
static void largestPalindrome(string s)
{
// string length
int l = s.Length;
// Map that marks the occurrence of a number
Dictionary m = new Dictionary();
for(int i = 0; i < 10; i++)
m[i] = 0;
for(int i = 0; i < l; i++)
m[s[i] - '0']++;
// Check the possibility of a
// palindromic number
if (possibility(m, l, s) == false)
{
Console.Write("Palindrome cannot be formed");
return;
}
// string array that stores the largest
// permuted palindromic number
char []largest = new char[l];
// Pointer of front
int front = 0;
// Greedily start from 9 to 0 and place the
// greater number in front and odd in the
// middle
for(int i = 9; i >= 0; i--)
{
// If the occurrence of number is odd
if ((m[i] & 1) != 0)
{
// Place one odd occurring number
// in the middle
largest[l / 2] = (char)(i + '0');
// Decrease the count
m[i]--;
// Place the rest of numbers greedily
while (m[i] > 0)
{
largest[front] = (char)(i + '0');
largest[l - front - 1] = (char)(i + '0');
m[i] -= 2;
front++;
}
}
else
{
// If all numbers occur even times,
// then place greedily
while (m[i] > 0)
{
// Place greedily at front
largest[front] = (char)(i + '0');
largest[l - front - 1] = (char)(i + '0');
// 2 numbers are placed, so
// decrease the count
m[i] -= 2;
// Increase placing position
front++;
}
}
}
// Print the largest string thus formed
for(int i = 0; i < l; i++)
{
Console.Write(largest[i]);
}
}
// Driver Code
public static void Main(string[] args)
{
string s = "313551";
largestPalindrome(s);
}
}
// This code is contributed by rutvik_56 输出：
531135
时间复杂度： O(n)，其中n是字符串的长度。