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22. Word Break

mediumAsked at Coupang

Decide whether a string can be segmented into dictionary words, mirroring how Coupang's Korean e-commerce search ranking segments user queries against an in-memory product-name lexicon.

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 dictionary words. Words can be reused.

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

Approaches

1. Recursion

Try every prefix and recurse on the remainder.

Time
O(2^n)
Space
O(n)
function dfs(s) {
  if (s.length === 0) return true;
  for (const w of wordDict)
    if (s.startsWith(w) && dfs(s.slice(w.length))) return true;
  return false;
}

Tradeoff:

2. Bottom-up DP

dp[i] = can prefix of length i be segmented. dp[i] is true if some j < i has dp[j] true and s[j..i] in dict.

Time
O(n^2)
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:

Coupang-specific tips

Coupang's Korean e-commerce search ranking segments user queries against an in-memory lexicon under tight latency; DP with a hashed lexicon is the canonical pattern for sub-millisecond segmentation.

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Output

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