通过只替换一个字符，使字符串字典顺序最小且非回文

给定一个只包含小写字母的回文字符串str ，任务是打印字典上最小的字符串，通过恰好替换一个字符，这样的字符串不是回文。
例子：

Input: str = “abccba” 
Output: “aaccba” 
Explanation: 
Lexicographically smallest non-palindromic string possible is “aaccba”, here we have replaced the second letter ‘b’ by an ‘a’ which makes it a non palindrome.
Input: str = “a” 
Output: -1 
Explanation: 
A single character is always a palindrome hence we cannot replace the value. Therefore the output is -1. 
 

方法：为了解决这个问题上面提到，我们将检查只有一半的字符串和替换哪些不是所有的字符“a”到字符“a”本身。这个问题的边缘情况是如果只有一个字符，我们将返回一个空字符串。否则，如果所有字符都相同，那么我们将仅用字符’b’ 替换最后一个字符。
下面是上述方法的实现：

C++

// C++ program to make the string
// lexicographically smallest non
// palindromic string by replacing
// exactly one character
 
#include 
using namespace std;
 
// Function to find the required string
string findStr(string S)
{
    // length of the string
    int n = S.size();
 
    // Iterate till half of the string
    for (int i = 0; i < n / 2; ++i) {
 
        // replacing a non 'a' char with 'a'
        if (S[i] != 'a') {
            S[i] = 'a';
 
            return S;
        }
    }
 
    // Check if there is no 'a' in string
    // we replace last char of string by 'b'
    S[n - 1] = 'b';
 
    // If the input is a single character
    return n < 2 ? " -1 " : S;
}
 
// Driver code
int main()
{
    string str = "a";
    cout << findStr(str) << endl;
 
    string str1 = "abccba";
    cout << findStr(str1) << endl;
 
    return 0;
}

Java

// Java program to make the string
// lexicographically smallest non
// palindromic string by replacing
// exactly one character
import java.util.*;
class GFG {
 
// Function to find the required string
static String findStr(String S)
{
    StringBuilder sb = new StringBuilder(S);
 
    // length of the string
    int n = sb.length();
 
    // Iterate till half of the string
    for (int i = 0; i < n / 2; ++i)
    {
 
        // replacing a non 'a' char with 'a'
        if (sb.charAt(i) != 'a')
        {
            sb.setCharAt(i, 'a');
 
            return sb.toString();
        }
    }
 
    // Check if there is no 'a' in string
    // we replace last char of string by 'b'
    sb.setCharAt(n - 1, 'b');
 
    // If the input is a single character
    return n < 2 ? " -1 " : sb.toString();
}
 
// Driver code
public static void main(String[] args)
{
    String str = "a";
    System.out.println(findStr(str));
 
    String str1 = "abccba";
    System.out.println(findStr(str1));
}
}
 
// This code is contributed by offbeat

Python3

# Python3 program to make the string
# lexicographically smallest non
# palindromic string by replacing
# exactly one character
 
# Function to find the required string
def findStr(S):
     
    S = list(S)
     
    # Length of the string
    n = len(S)
     
    # Iterate till half of the string
    for i in range(0, n // 2):
         
        # Replacing a non 'a' char with 'a'
        if S[i] != 'a':
            S[i] = 'a'
            return (''.join(S))
     
    # Check if there is no 'a' in string
    # we replace last char of string by 'b'
    S[n - 1] = 'b'
     
    # If the input is a single character
    if n < 2:
        return '-1'
    else:
        return (''.join(S))
     
# Driver Code
if __name__=='__main__':
     
    str1 = 'a'
    print(findStr(str1))
     
    str2 = 'abccba'
    print(findStr(str2))
 
# This code is contributed by rutvik_56

C#

// C# program to make the string
// lexicographically smallest non
// palindromic string by replacing
// exactly one character
using System;
 
class GFG{
 
// Function to find the required string
static String findStr(char []S)
{
 
    // Length of the string
    int n = S.Length;
 
    // Iterate till half of the string
    for(int i = 0; i < n / 2; ++i)
    {
        
       // Replacing a non 'a' char with 'a'
       if (S[i] != 'a')
       {
           S[i] = 'a';
           return new String(S);
       }
    }
 
    // Check if there is no 'a' in string
    // we replace last char of string by 'b'
    S[n - 1] = 'b';
 
    // If the input is a single character
    return n < 2 ? " -1 " : new String(S);
}
 
// Driver code
public static void Main(String[] args)
{
    String str = "a";
    Console.WriteLine(findStr(str.ToCharArray()));
 
    String str1 = "abccba";
    Console.WriteLine(findStr(str1.ToCharArray()));
}
}
 
// This code is contributed by 29AjayKumar

Javascript

 
// Javascript program to make the string
// lexicographically smallest non
// palindromic string by replacing
// exactly one character
 
// Function to find the required string
function findStr(S)
{
    // length of the string
    var n = S.length;
 
    // Iterate till half of the string
    for (var i = 0; i < n / 2; ++i) {
 
        // replacing a non 'a' char with 'a'
        if (S[i] != 'a') {
            S[i] = 'a';
 
            return S.join("");;
        }
    }
 
    // Check if there is no 'a' in string
    // we replace last char of string by 'b'
    S[n - 1] = 'b';
 
    // If the input is a single character
    return n < 2 ? " -1 " : S.join("");;
}
 
// Driver code
 
var str = "a";
document.write( findStr(Array.from(str)) + "<br>");
var str1 = "abccba";
document.write( findStr(Array.from(str1)));
 

输出：

-1 
aaccba

时间复杂度： O(N)

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