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22. Word Break

mediumAsked at Grab

Decide whether a string can be segmented into dictionary words — Grab uses this as a DP-on-strings warm-up.

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.

Constraints

  • 1 <= s.length <= 300
  • 1 <= wordDict.length <= 1000
  • 1 <= wordDict[i].length <= 20

Examples

Example 1

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

Example 2

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

Approaches

1. Plain recursion

Try every dictionary word as a prefix; recurse on the suffix.

Time
O(2^n)
Space
O(n)
function can(s) {
  if (s === '') return true;
  for (const w of wordDict)
    if (s.startsWith(w) && can(s.slice(w.length))) return true;
  return false;
}

Tradeoff:

2. Bottom-up DP

dp[i] = true if s[0..i] is segmentable; fill left-to-right by checking each ending split.

Time
O(n^2 * k)
Space
O(n)
function wordBreak(s, wordDict) {
  const set = 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] && set.has(s.slice(j, i))) { dp[i] = true; break; }
    }
  }
  return dp[s.length];
}

Tradeoff:

Grab-specific tips

Grab interviewers expect you to memoize from the start — frame as parsing concatenated SEA service codes (food + ride + wallet) into known prefixes.

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Output

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