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Problem solution · Java

Color the Triangle Red

Color the Triangle Red: a Java solution using direct simulation. Learn the idea, check the complexity, and read the full code, with credit to walkccc LeetCode Solutions.

Technique
Direct simulation
Source
walkccc LeetCode Solutions
Length
32 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

Direct simulation

For Color the Triangle Red, the implementation follows the problem’s operations directly while maintaining only the state needed for the next decision.

  1. Translate each rule into one explicit state update.
  2. Maintain the invariant after every processed item.
  3. Return the accumulated state once all relevant input has been handled.

Code notes

  • 32 lines of Java from the credited upstream file 2647.java.
  • The implementation visibly relies on sequence storage.
  • 4 loop blocks detected.

Complexity

Count the number and nesting of passes over the input, then include the maintained containers in the memory estimate.

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 codeColor the Triangle Red · JavaJava
Use this to learn the idea, then write your own version.
class Solution {  public int[][] colorRed(int n) {    List<int[]> ans = new ArrayList<>();    final int tipSize = n % 4;    // The tip of the triangle is always painted red.    if (tipSize >= 1)      ans.add(new int[] {1, 1});     // Paint the rightmost and the leftmost elements at the following rows.    for (int i = 2; i <= tipSize; ++i) {      ans.add(new int[] {i, 1});      ans.add(new int[] {i, 2 * i - 1});    }     // Paint the 4-row chunks.    for (int i = tipSize + 1; i < n; i += 4) {      // Fill the first row of the chunk.      ans.add(new int[] {i, 1});      // Fill the second row.      for (int j = 1; j <= i; ++j)        ans.add(new int[] {i + 1, 2 * j + 1});      // Fill the third row.      ans.add(new int[] {i + 2, 2});      // Fill the fourth row.      for (int j = 0; j <= i + 2; ++j)        ans.add(new int[] {i + 3, 2 * j + 1});    }     return ans.stream().toArray(int[][] ::new);  }}

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