给定两个字符串A和B ,任务是通过以下操作计算构造字符串B所需的最少操作数:
- 选择字符串A的子序列。
- 将子序列附加到新形成的字符串(最初为空)。
打印所需的最少操作数。如果无法通过应用给定的操作使新字符串等于B ,则打印-1 。
例子:
Input: A = “abc”, B = “abac”
Output: 2
Explanation:
Initially, C = “”.
Step 1: Select subsequence “ab” from string A and append it to the empty string C, i.e. C = “ab”.
Step 2: Select subsequence “ac” from string A and append it to the end of string C, i.e. C = “abac”.
Now, the string C is same as string B.
Therefore, count of operations required is 2.
Input: A = “geeksforgeeks”, B = “programming”
Output: -1
方法:按照以下步骤解决此问题:
- 初始化地图绘制目前在各自的指数的字符串中的字符。
- 对于字符串A 中的每个字符,跟踪它的所有出现。
- 初始化一个变量,比如ans ,以存储所需的操作计数。由于操作次数必须大于1 ,因此设置ans = 1 。
- 遍历字符串B和检查的字符,如果字符是存在于字符串A或不使用地图。
- 最后,最大化从字符串A 中为每个操作选择的子序列的长度。
- 最后,打印所需的最少操作。
下面是上述方法的实现:
C++
// C++ program for the above approach
#include
using namespace std;
// Function to count the minimum
// subsequences of a string A required
// to be appended to obtain the string B
void countminOpsToConstructAString(string A,
string B)
{
// Size of the string
int N = A.length();
int i = 0;
// Maps characters to their
// respective indices
map > mp;
// Insert indices of characters
// into the sets
for (i = 0; i < N; i++) {
mp[A[i]].insert(i);
}
// Stores the position of the last
// visited index in the string A.
// Initially set it to -1.
int previous = -1;
// Stores the required count
int ans = 1;
// Iterate over the characters of B
for (i = 0; i < B.length(); i++) {
char ch = B[i];
// If the character in B is
// not present in A, return -1
if (mp[ch].size() == 0) {
cout << -1;
return;
}
// Fetch the next index from B[i]'s set
auto it = mp[ch].upper_bound(previous);
// If the iterator points to
// the end of that set
if (it == mp[ch].end()) {
previous = -1;
ans++;
--i;
continue;
}
// If it doesn't point to the
// end, update previous
previous = *it;
}
// Print the answer
cout << ans;
}
// Driver Code
int main()
{
string A = "abc", B = "abac";
countminOpsToConstructAString(A, B);
return 0;
}
Python3
# Python3 program for the above approac
from bisect import bisect_right
# Function to count the minimum
# subsequences of a A required
# to be appended to obtain the B
def countminOpsToConstructAString(A, B):
# Size of the string
N = len(A)
i = 0
# Maps characters to their
# respective indices
mp = [[] for i in range(26)]
# Insert indices of characters
# into the sets
for i in range(N):
mp[ord(A[i]) - ord('a')].append(i)
# Stores the position of the last
# visited index in the A.
# Initially set it to -1.
previous = -1
# Stores the required count
ans, i = 1, 0
# Iterate over the characters of B
while i < len(B):
ch = B[i]
# If the character in B is
# not present in A, return -1
if (len(mp[ord(ch) - ord('a')]) == 0):
print(-1)
return
# Fetch the next index from B[i]'s set
it = bisect_right(mp[ord(ch) - ord('a')], previous)
# If the iterator points to
# the end of that set
if (it == len(mp[ord(ch) - ord('a')])):
previous = -1
ans += 1
# i -= 1
continue
# If it doesn't poto the
# end, update previous
previous = mp[ord(ch) - ord('a')][it]
i += 1
# Prthe answer
print (ans)
# Driver Code
if __name__ == '__main__':
A, B = "abc", "abac"
countminOpsToConstructAString(A, B)
# This code is contributed by mohit kumar 29.
输出:
2
时间复杂度: O(N * logN)
辅助空间: O(N)
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