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Problem solution · Java

Number of Ways to Wear Different Hats to Each Other

Number of Ways to Wear Different Hats to Each Other: a Java solution using dynamic programming. Learn the idea, check the complexity, and read the full code, with credit to walkccc LeetCode Solutions.

Technique
Dynamic programming
Source
walkccc LeetCode Solutions
Length
31 lines
Start with the idea.

Try the problem first. If you get stuck, read the approach below, then write your own solution. The full code is at the bottom.

Approach

Dynamic programming

For Number of Ways to Wear Different Hats to Each Other, the implementation records answers for smaller states and reuses them to build the requested result without repeating work.

  1. Define precisely what one DP state represents.
  2. Establish the base cases before transitions are evaluated.
  3. Process states in dependency order and combine only already-known values.

Code notes

  • 31 lines of Java from the credited upstream file 1434-3.java.
  • The implementation visibly relies on sequence storage, cached states.
  • 6 loop blocks detected.

Complexity

Multiply the number of reachable states by the work performed for each transition, then include the stored state table in memory usage.

Check the problem constraints before deciding whether this complexity will pass.

Source

Code and credit

This code comes from walkccc LeetCode Solutions by P.-Y. Chen (walkccc) and is used under the MIT licence.

Open problem ↗Original source ↗
Full codeNumber of Ways to Wear Different Hats to Each Other · JavaJava
Use this to learn the idea, then write your own version.
class Solution {  public int numberWays(List<List<Integer>> hats) {    final int MOD = 1_000_000_007;    final int nHats = 40;    final int nPeople = hats.size();    final int nAssignments = 1 << nPeople;    List<Integer>[] hatToPeople = new List[nHats + 1];    // dp[i] := the number of ways to assign the hats so far to people, where i    // is the bitmask of the current assignment    int[] dp = new int[nAssignments];    dp[0] = 1;     for (int i = 1; i <= nHats; ++i)      hatToPeople[i] = new ArrayList<>();     for (int i = 0; i < nPeople; ++i)      for (final int hat : hats.get(i))        hatToPeople[hat].add(i);     for (int h = 1; h <= nHats; ++h)      for (int j = nAssignments - 1; j >= 0; --j)        for (final int p : hatToPeople[h])          if ((j >> p & 1) == 1) {            dp[j] += dp[j ^ 1 << p];            dp[j] %= MOD;          }     return dp[nAssignments - 1];  }}

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