LeetCode 139: Word Break, Starting from Prefix Reachability

Tue, 22 Sep 2026 12:00:00 +0800

Problem

Input and Output

Input: a string s and a list of strings wordDict
Every element in wordDict is an available word, and the same word may be reused
Output: return True if s can be split into one or more dictionary words; otherwise, return False
The split must cover the entire string without skipping or reordering characters

Examples

Input: s = "leetcode", wordDict = ["leet", "code"]
Output: True
Explanation: "leetcode" can be split into "leet" + "code"

Input: s = "applepenapple", wordDict = ["apple", "pen"]
Output: True
Explanation: "apple" + "pen" + "apple"; the same word may be reused

Input: s = "catsandog", wordDict = ["cats", "dog", "sand", "and", "cat"]
Output: False
Explanation: some prefixes can be formed, but the split cannot cover the entire string

Constraints

1 <= s.length <= 300
1 <= wordDict.length <= 1000
1 <= wordDict[i].length <= 20
s and wordDict[i] contain only lowercase English letters
All strings in wordDict are unique

Step 1: Make the Completed Prefix Visible

Start with one concrete question. For s = "leetcode", before deciding whether the whole string can be split, can we record how much of it has already been completed?

LeetCode 139 on Jeanphilo Blog

LeetCode 139: Word Break, Starting from Prefix Reachability

Problem

Input and Output

Examples

Constraints

Step 1: Make the Completed Prefix Visible