Leetcode #524: Longest Word in Dictionary through Deleting
In this guide, we solve Leetcode #524 Longest Word in Dictionary through Deleting in Python and focus on the core idea that makes the solution efficient.
You will see the intuition, the step-by-step method, and a clean Python implementation you can use in interviews.
Problem Statement
Given a string s and a string array dictionary, return the longest string in the dictionary that can be formed by deleting some of the given string characters. If there is more than one possible result, return the longest word with the smallest lexicographical order.
Quick Facts
- Difficulty: Medium
- Premium: No
- Tags: Array, Two Pointers, String, Sorting
Intuition
The constraints hint that we can reason about two ends of the data at once, which is perfect for a two-pointer scan.
Moving one pointer at a time keeps the invariant intact and avoids nested loops.
Approach
Place pointers at the left and right ends and move them based on the comparison or target condition.
This yields a clean linear pass after any required sorting.
Steps:
- Set left and right pointers.
- Move a pointer based on the condition.
- Update the best answer while scanning.
Example
Input: s = "abpcplea", dictionary = ["ale","apple","monkey","plea"]
Output: "apple"
Python Solution
class Solution:
def findLongestWord(self, s: str, dictionary: List[str]) -> str:
def check(s: str, t: str) -> bool:
m, n = len(s), len(t)
i = j = 0
while i < m and j < n:
if s[i] == t[j]:
i += 1
j += 1
return i == m
ans = ""
for t in dictionary:
if check(t, s) and (len(ans) < len(t) or (len(ans) == len(t) and ans > t)):
ans = t
return ans
Complexity
The time complexity is , where is the length of the string list, and and are the lengths of string and the average length of strings in the list, respectively. The space complexity is .
Edge Cases and Pitfalls
Watch for boundary values, empty inputs, and duplicate values where applicable. If the problem involves ordering or constraints, confirm the invariant is preserved at every step.
Summary
This Python solution focuses on the essential structure of the problem and keeps the implementation interview-friendly while meeting the constraints.