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26. Word Break

mediumAsked at Booking

Determine if a string can be segmented using a dictionary of valid words — Booking's NLP pipeline applies the same DP-based segmentation when parsing free-text destination queries into canonical city and region tokens.

By Alex Chen, Founder, InterviewChamp.AI · Last verified

Problem

Given a string s and a dictionary of strings wordDict, return true if s can be segmented into a space-separated sequence of one or more dictionary words. Words in the dictionary may be reused.

Constraints

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

Examples

Example 1

Input
s = "leetcode", wordDict = ["leet","code"]
Output
true

Explanation: "leet" + "code" = "leetcode".

Example 2

Input
s = "applepenapple", wordDict = ["apple","pen"]
Output
true

Example 3

Input
s = "catsandog", wordDict = ["cats","dog","sand","and","cat"]
Output
false

Approaches

1. Recursive with memoization

Try every word at the current position; if it matches and the remainder also succeeds, return true. Cache results by index.

Time
O(n^2 * m)
Space
O(n)
function wordBreak(s, wordDict) {
  const wordSet = new Set(wordDict);
  const memo = new Map();

  function dp(start) {
    if (start === s.length) return true;
    if (memo.has(start)) return memo.get(start);

    for (let end = start + 1; end <= s.length; end++) {
      if (wordSet.has(s.slice(start, end)) && dp(end)) {
        memo.set(start, true);
        return true;
      }
    }

    memo.set(start, false);
    return false;
  }

  return dp(0);
}

Tradeoff:

2. Bottom-up DP

dp[i] = true if s[0..i) can be segmented. For each position, check all words that end at i.

Time
O(n^2 * m)
Space
O(n)
function wordBreak(s, wordDict) {
  const wordSet = new Set(wordDict);
  const dp = new Array(s.length + 1).fill(false);
  dp[0] = true;

  for (let i = 1; i <= s.length; i++) {
    for (let j = 0; j < i; j++) {
      if (dp[j] && wordSet.has(s.slice(j, i))) {
        dp[i] = true;
        break;
      }
    }
  }

  return dp[s.length];
}

Tradeoff:

Booking-specific tips

Booking processes queries in 43 languages — they appreciate candidates who mention that the word-set approach generalises to multi-language dictionaries and ask how you'd handle overlapping word boundaries (the DP handles it naturally).

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Output

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