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This approach uses two pointers to scan both strings, one pointer for each string. The idea is to attempt to match characters of t with those in s. A character from t is successfully matched if it is equal to the current character in s. We iterate through s, moving the pointer for each character matched, while the pointer for t only moves if a match is found. Once we've iterated through s, we determine how many characters are left unmatched in t, which is the number of characters needed to be appended to s.
Time Complexity: O(n + m) where n is the length of s and m is the length of t.
Space Complexity: O(1) as no additional space is used.
1def minCharsToAppend(s, t):
2 s_len, t_len = len(s), len(t)
3 s_index, t_index = 0, 0
4
The Python solution defines the function minCharsToAppend where two indices are used to iterate over the strings. For every matched character, the index of t moves forward. The number of unmatched characters in t is returned as the count of characters to append.
This approach uses dynamic programming to precompute the next occurrence of each character for every position in the string s. By doing so, we quickly identify the existence and location of each character from t in s. We build a 2D array next where next[i][j] represents the smallest index >= i that holds the character j in s. This helps in efficiently determining the position of characters and the necessary additions to the end of the string.
Time Complexity: O(n + m + n * ALPHABET_SIZE) due to preprocessing each character for positions.
Space Complexity: O(n * ALPHABET_SIZE) used by the 2D array next where n is length of s.
1def minCharsToAppend(s, t):
2 s_len, t_len = len(s), len(t)
3 next_pos = [[s_len] * 26 for _ in range(s_len + 1)]
4
5 last = [s_len] * 26
6 for i in range(s_len - 1, -1, -1):
7 last[ord(s[i]) - ord('a')] = i
8 for j in range(26):
9 next_pos[i][j] = last[j]
10
11 pos = 0
12 append_count = 0
13 for c in t:
14 if next_pos[pos][ord(c) - ord('a')] == s_len:
15 append_count += (t_len - t.index(c))
16 break
17 pos = next_pos[pos][ord(c) - ord('a')] + 1
18
19 return append_count
20
21# Testing the function
22s = "coaching"
23t = "coding"
24print(minCharsToAppend(s, t))Python's solution defines the next_pos array, precomputing where each character appears next after each index and using this data to decide how many appended characters are necessary for t to be its subsequence.