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leetcode - 1055. Shortest Way to Form String

news 来源：原创 2025/6/18 4:03:03

Description

A subsequence of a string is a new string that is formed from the original string by deleting some (can be none) of the characters without disturbing the relative positions of the remaining characters. (i.e., “ace” is a subsequence of “abcde” while “aec” is not).

Given two strings source and target, return the minimum number of subsequences of source such that their concatenation equals target. If the task is impossible, return -1.

Example 1:

Input: source = "abc", target = "abcbc"
Output: 2
Explanation: The target "abcbc" can be formed by "abc" and "bc", which are subsequences of source "abc".

Example 2:

Input: source = "abc", target = "acdbc"
Output: -1
Explanation: The target string cannot be constructed from the subsequences of source string due to the character "d" in target string.

Example 3:

Input: source = "xyz", target = "xzyxz"
Output: 3
Explanation: The target string can be constructed as follows "xz" + "y" + "xz".

Constraints:

1 <= source.length, target.length <= 1000
source and target consist of lowercase English letters.

Solution

DP (TLE)

Use dp[i] to denote the minimum number of subsequence we need to form target[:i], then the transformation equation is:
$\min(dp[k - 1]) + 1, \forall k \; \text{that }target[k:i] \text{ is a subsequence of source}$

Time complexity: $o(target.len^2*source.len)=o(n^3)$
Space complexity: $o (t a r g e t . l e n)$

Greedy

Go through target, and if the current character is not the same as source, move the pointer in source one step forward. Start over when it’s the end of source.

Time complexity: $o (t a r g e t . l e n * so u rce . l e n)$
Space complexity: $o (1)$

Code

DP (TLE)

class Solution:def shortestWay(self, source: str, target: str) -> int:def is_subsequence(source: str, target: str) -> bool:i, j = 0, 0while i < len(source) and j < len(target):if source[i] == target[j]:i += 1j += 1else:i += 1return j >= len(target)if set(target) - set(source):return -1dp = [i + 1 for i in range(len(target))]for i in range(1, len(target)):for k in range(i + 1):if is_subsequence(source, target[k: i + 1]):dp[i] = min(dp[i], 1 + (dp[k - 1] if k - 1 >= 0 else 0))return dp[-1]

Greedy

class Solution:def shortestWay(self, source: str, target: str) -> int:if set(target) - set(source):return -1source_index, target_index = 0, 0res = 0while target_index < len(target):if target[target_index] == source[source_index]:target_index += 1source_index += 1if source_index == len(source):source_index = 0res += 1return res + (1 if source_index != 0 else 0)