Find And Replace in String Python Solution

Tested Python solution for LeetCode 833 with 16 pytest cases. Generate a practice environment with lcpy.

LeetCode 833, Medium. Topics: Array, Hash Table, String, Sorting. View on LeetCode.

Generate this problem as a practice environment: tested reference solution, 16 parametrized pytest cases, and a playground notebook:

lcpy gen -n 833   # by problem number
lcpy gen -s find_and_replace_in_string   # by problem name

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Problem

You are given a 0-indexed string s that you must perform k replacement operations on. The replacement operations are given as three 0-indexed parallel arrays, indices, sources, and targets, all of length k.

To complete the ith replacement operation:

• Check if the substring sources[i] occurs at index indices[i] in the original string s.
• If it does not occur, do nothing.
• Otherwise if it does occur, replace that substring with targets[i].

For example, if s = "abcd", indices[i] = 0, sources[i] = "ab", and targets[i] = "eee", then the result of this replacement will be "eeecd".

All replacement operations must occur simultaneously, meaning the replacement operations should not affect the indexing of each other. The testcases will be generated such that the replacements will not overlap.

• For example, a testcase with s = "abc", indices = [0, 1], and sources = ["ab","bc"] will not be generated because the "ab" and "bc" replacements overlap.

Return the resulting string after performing all replacement operations on s.

A substring is a contiguous sequence of characters in a string.

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Examples

Input: s = "abcd", indices = [0, 2], sources = ["a", "cd"], targets = ["eee", "ffff"]
Output: "eeebffff"
Explanation:
"a" occurs at index 0 in s, so we replace it with "eee".
"cd" occurs at index 2 in s, so we replace it with "ffff".

Input: s = "abcd", indices = [0, 2], sources = ["ab","ec"], targets = ["eee","ffff"]
Output: "eeecd"
Explanation:
"ab" occurs at index 0 in s, so we replace it with "eee".
"ec" does not occur at index 2 in s, so we do nothing.

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Constraints

• 1 <= s.length <= 1000
• k == indices.length == sources.length == targets.length
• 1 <= k <= 100
• 0 <= indices[i] < s.length
• 1 <= sources[i].length, targets[i].length <= 50
• s consists of only lowercase English letters.
• sources[i] and targets[i] consist of only lowercase English letters.

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Solution

Reference implementation from solution.py on GitHub, full suite in test_solution.py:

class Solution:
    # Time: O(n + sum(len(sources[i]) + len(targets[i])))
    # Space: O(n)
    def find_replace_string(
        self, s: str, indices: list[int], sources: list[str], targets: list[str]
    ) -> str:
        match_at: dict[int, int] = {}
        for i, idx in enumerate(indices):
            if s.startswith(sources[i], idx):
                match_at[idx] = i

        pieces: list[str] = []
        i = 0
        while i < len(s):
            j = match_at.get(i)
            if j is None:
                pieces.append(s[i])
                i += 1
            else:
                pieces.append(targets[j])
                i += len(sources[j])
        return "".join(pieces)

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Complexity

Time	Space
O(n + sum(len(sources[i]) + len(targets[i])))	O(n)

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