Leetcode-Combination Series

Usually, when it comes to combination problems, the most naive way to solve the problem is generate all possible solutions. And, we use depth-first-search(DFS) or called backtracking technique.

For backtracking, when we do time and space complexity analysis, they follow a certain pattern:

T: O(n**m), n raise to power of m, where n represents at most we have n branches can search for current state and m is maximum depth of recursion tree.

Note: O(n**m) will be upper bound of time complexity. For some cases, if we generate all possible combinations or partitions , given an integer or a string, running time will be O(2**n), lower than upper bound.

S:O(m), where m is maximum depth of recursion tree. (if we don’t consider space of storing answer)

  • 39. Combination Sum
  • 131. Palindrome Partitioning
  • 291. Word Pattern II



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Yunrui Li

I’m Taiwanese expat. I’v worked in Singapore as data scientist after graduation from Taiwan and currently I work in Amsterdam as machine learning engineer.