给定长度为N 的字符串str和整数数组arr[] ，对于数组元素arr[i](基于 1 的索引) ，反转索引[arr[i], N – arr[i] + 1] 中的子字符串。任务是在每次反转后打印字符串。
例子：
Input: str = “GeeksforGeeks”, arr[] = {2}
Output: GkeeGrofskees
Explanation:
For first element of the array is 2:
Reverse the substring (2, 12). Now the updated string is “GkeeGrofskees”.
Input: str = “abcdef”, arr[] = {1, 2, 3}
Output: fbdcea
Explanation:
For first element of the array is 1:
Reverse the substring (1, 6). Now the updated string is “fedcba”.
For second element of the array is 2:
Reverse the substring (2, 5). Now the updated string is “fbcdea”.
For third element of the array is 3:
Reverse the substring (3, 4). Now the updated string is “fbdcea”.
朴素的方法：最简单的方法是遍历给定的数组，并为每个数组元素arr[i]反转子字符串{s[arr[i]], … s[N – arr[i] + 1]}并打印结果非常更新后获得的字符串。
时间复杂度： O(N * K)，其中 N 是字符串的长度，K 是反转子字符串的最大长度。
辅助空间： O(1)
有效的方法：可以通过跟踪索引处的任何字符已被反转的次数来优化上述方法。如果反转的次数是偶数，那么字符会回到原来的位置，所以不会有任何变化，如果反转的次数是奇数，则必须交换字符。以下是步骤：
- 初始化数组count[]以存储字符串 的任何索引处的反转次数。
- 遍历给定的数组arr[]并将索引count[arr[i]]的计数增加1 。
- 现在，使用变量i在范围[1, N/2] 上遍历数组count[]并执行以下操作：
- 将当前索引处的元素更新为当前索引和上一个索引的总和。
- 现在，如果当前元素count[i]是奇数，则交换str[i]和str[N – i + 1] 。
- 在上述步骤之后打印字符串。
下面是上述方法的实现：
C++
// C++ program for the above approach
#include
using namespace std;
// Function to perform the reversal
// operation on the given string
string modifyString(int A[], string str,
int K)
{
// Size of string
int N = str.size();
// Stores the count of indices
int count[N + 1] = { 0 };
// Count the positions where
// reversals will begin
for (int i = 0; i < K; i++) {
count[A[i]]++;
}
for (int i = 1; i <= N / 2; i++) {
// Store the count of reversals
// beginning at position i
count[i] = count[i] + count[i - 1];
// Check if the count[i] is
// odd the swap the character
if (count[i] & 1) {
swap(str[i - 1], str[N - i]);
}
}
// Return the updated string
return str;
}
// Driver Code
int main()
{
// Given string str
string str = "abcdef";
// Given array of reversing index
int arr[] = { 1, 2, 3 };
int K = sizeof(arr) / sizeof(arr[0]);
// Function Call
cout << modifyString(arr, str, K);
return 0;
} Java
// Java program for the above approach
import java.util.*;
class GFG{
// Function to perform the reversal
// operation on the given String
static String modifyString(int A[], String str,
int K)
{
// Size of String
int N = str.length();
// Stores the count of indices
int count[] = new int[N + 1];
// Count the positions where
// reversals will begin
for(int i = 0; i < K; i++)
{
count[A[i]]++;
}
for(int i = 1; i <= N / 2; i++)
{
// Store the count of reversals
// beginning at position i
count[i] = count[i] + count[i - 1];
// Check if the count[i] is
// odd the swap the character
if ((count[i] & 1) > 0)
{
str = swap(str, i - 1, N - i);
}
}
// Return the updated String
return str;
}
// Swap char of a string
static String swap(String str, int i, int j)
{
char ch[] = str.toCharArray();
char temp = ch[i];
ch[i] = ch[j];
ch[j] = temp;
return String.valueOf(ch);
}
// Driver Code
public static void main(String[] args)
{
// Given String str
String str = "abcdef";
// Given array of reversing index
int arr[] = { 1, 2, 3 };
int K = arr.length;
// Function Call
System.out.print(modifyString(arr, str, K));
}
}
// This code is contributed by Amit KatiyarPython3
# Python3 program for the above approach
# Function to perform the reversal
# operation on the given string
def modifyString(A, str, K):
# Size of string
N = len(str)
# Stores the count of indices
count = [0] * (N + 1)
# Count the positions where
# reversals will begin
for i in range(K):
count[A[i]] += 1
for i in range(1, N // 2 + 1):
# Store the count of reversals
# beginning at position i
count[i] = count[i] + count[i - 1]
# Check if the count[i] is
# odd the swap the character
if (count[i] & 1):
str[i - 1], str[N - i] = str[N - i], str[i - 1]
# Return the updated string
return "".join(str)
# Driver Code
if __name__ == '__main__':
# Given str
str1 = "abcdef"
str = [i for i in str1]
# Given array of reversing index
arr = [ 1, 2, 3 ]
K = len(arr)
# Function Call
print(modifyString(arr, str, K))
# This code is contributed by mohit kumar 29C#
// C# program for the above approach
using System;
class GFG{
// Function to perform the reversal
// operation on the given String
static String modifyString(int []A, String str,
int K)
{
// Size of String
int N = str.Length;
// Stores the count of indices
int []count = new int[N + 1];
// Count the positions where
// reversals will begin
for(int i = 0; i < K; i++)
{
count[A[i]]++;
}
for(int i = 1; i <= N / 2; i++)
{
// Store the count of reversals
// beginning at position i
count[i] = count[i] + count[i - 1];
// Check if the count[i] is
// odd the swap the character
if ((count[i] & 1) > 0)
{
str = swap(str, i - 1, N - i);
}
}
// Return the updated String
return str;
}
// Swap char of a string
static String swap(String str, int i, int j)
{
char []ch = str.ToCharArray();
char temp = ch[i];
ch[i] = ch[j];
ch[j] = temp;
return String.Join("", ch);
}
// Driver Code
public static void Main(String[] args)
{
// Given String str
String str = "abcdef";
// Given array of reversing index
int []arr = { 1, 2, 3 };
int K = arr.Length;
// Function Call
Console.Write(modifyString(arr, str, K));
}
}
// This code is contributed by Amit Katiyar输出：
fbdcea
时间复杂度： O(N + K)，其中 N 是字符串的长度，K 是反转子字符串的最大长度。
辅助空间： O(N)
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