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Posts tagged as “DFS”

花花酱 LeetCode 212. Word Search II

Given a 2D board and a list of words from the dictionary, find all words in the board.

Each word must be constructed from letters of sequentially adjacent cell, where “adjacent” cells are those horizontally or vertically neighboring. The same letter cell may not be used more than once in a word.


board = [
words = ["oath","pea","eat","rain"]

Output: ["eat","oath"]


  1. All inputs are consist of lowercase letters a-z.
  2. The values of words are distinct.

Solution 1: DFS

Time complexity: O(sum(m*n*4^l))
Space complexity: O(l)


Solution 2: Trie

Store all the words into a trie, search the board using DFS, paths must be in the trie otherwise there is no need to explore.

Time complexity: O(sum(l) + 4^max(l))
space complexity: O(sum(l) + l)


花花酱 LeetCode 90. Subsets II

Given a collection of integers that might contain duplicates, nums, return all possible subsets (the power set).

Note: The solution set must not contain duplicate subsets.


Solution: DFS

The key to this problem is how to remove/avoid duplicates efficiently.

For the same depth, among the same numbers, only the first number can be used.

Time complexity: O(2^n * n)
Space complexity: O(n)


花花酱 LeetCode 851. Loud and Rich

In a group of N people (labelled 0, 1, 2, ..., N-1), each person has different amounts of money, and different levels of quietness.

For convenience, we’ll call the person with label x, simply “person x“.

We’ll say that richer[i] = [x, y] if person x definitely has more money than person y.  Note that richer may only be a subset of valid observations.

Also, we’ll say quiet[x] = q if person x has quietness q.

Now, return answer, where answer[x] = y if y is the least quiet person (that is, the person y with the smallest value of quiet[y]), among all people who definitely have equal to or more money than person x.

Example 1:

Input: richer = [[1,0],[2,1],[3,1],[3,7],[4,3],[5,3],[6,3]], quiet = [3,2,5,4,6,1,7,0]
Output: [5,5,2,5,4,5,6,7]
answer[0] = 5.
Person 5 has more money than 3, which has more money than 1, which has more money than 0.
The only person who is quieter (has lower quiet[x]) is person 7, but
it isn't clear if they have more money than person 0.

answer[7] = 7.
Among all people that definitely have equal to or more money than person 7
(which could be persons 3, 4, 5, 6, or 7), the person who is the quietest (has lower quiet[x])
is person 7.

The other answers can be filled out with similar reasoning.


  1. 1 <= quiet.length = N <= 500
  2. 0 <= quiet[i] < N, all quiet[i] are different.
  3. 0 <= richer.length <= N * (N-1) / 2
  4. 0 <= richer[i][j] < N
  5. richer[i][0] != richer[i][1]
  6. richer[i]‘s are all different.
  7. The observations in richer are all logically consistent.

Solution: DFS + Memoization

For person i , remember the quietest person who is richer than person i.

Time complexity: O(n^2)
Space complexity: O(n)


花花酱 LeetCode 417. Pacific Atlantic Water Flow

Given an m x n matrix of non-negative integers representing the height of each unit cell in a continent, the “Pacific ocean” touches the left and top edges of the matrix and the “Atlantic ocean” touches the right and bottom edges.

Water can only flow in four directions (up, down, left, or right) from a cell to another one with height equal or lower.

Find the list of grid coordinates where water can flow to both the Pacific and Atlantic ocean.


  1. The order of returned grid coordinates does not matter.
  2. Both m and n are less than 150.


Given the following 5x5 matrix:

  Pacific ~   ~   ~   ~   ~ 
       ~  1   2   2   3  (5) *
       ~  3   2   3  (4) (4) *
       ~  2   4  (5)  3   1  *
       ~ (6) (7)  1   4   5  *
       ~ (5)  1   1   2   4  *
          *   *   *   *   * Atlantic


[[0, 4], [1, 3], [1, 4], [2, 2], [3, 0], [3, 1], [4, 0]] (positions with parentheses in above matrix).

Solution: DFS/BFS

Be careful with the input range, easy to TLE with a naive implementation. Have to search from the boards.

Time complexity: O(mn)
Space complexity: O(mn)




花花酱 LeetCode 996. Number of Squareful Arrays

Given an array A of non-negative integers, the array is squareful if for every pair of adjacent elements, their sum is a perfect square.

Return the number of permutations of A that are squareful.  Two permutations A1 and A2 differ if and only if there is some index i such that A1[i] != A2[i].

Example 1:

Input: [1,17,8]
Output: 2
[1,8,17] and [17,8,1] are the valid permutations.

Example 2:

Input: [2,2,2]
Output: 1


  1. 1 <= A.length <= 12
  2. 0 <= A[i] <= 1e9

Solution1: DFS

Try all permutations with pruning.

Time complexity: O(n!)
Space complexity: O(n)


Solution 2: DP Hamiltonian Path

dp[s][i] := # of ways to reach state s (binary mask of nodes visited) that ends with node i

dp[s | (1 << j)][j] += dp[s][i] if g[i][j]

Time complexity: O(n^2*2^n)
Space complexity: O(2^n)


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