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Problem solution · Java

Length of Longest V-Shaped Diagonal Segment

Length of Longest V-Shaped Diagonal Segment: a Java solution using depth-first search. Learn the idea, check the complexity, and read the full code, with credit to walkccc LeetCode Solutions.

Technique
Depth-first search
Source
walkccc LeetCode Solutions
Length
48 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

Depth-first search

For Length of Longest V-Shaped Diagonal Segment, the implementation follows one branch at a time, making it suitable for components, trees, backtracking, or dependency exploration.

  1. Define the state carried into one recursive or stack frame.
  2. Mark or choose the current state before exploring children.
  3. Combine child results or undo the choice when the branch finishes.

Code notes

  • 48 lines of Java from the credited upstream file 3459.java.
  • The implementation visibly relies on sequence storage.
  • 3 loop blocks detected, together with recursive traversal.

Complexity

Count unique states for graph traversal; for backtracking, count the branching factor and maximum depth.

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 codeLength of Longest V-Shaped Diagonal Segment · JavaJava
Use this to learn the idea, then write your own version.
class Solution {  public int lenOfVDiagonal(int[][] grid) {    final int m = grid.length;    final int n = grid[0].length;    Integer[][][][][] mem = new Integer[m][n][2][2][4];     int ans = 0;     for (int i = 0; i < m; ++i)      for (int j = 0; j < n; ++j)        if (grid[i][j] == 1)          for (int d = 0; d < 4; ++d) {            final int dx = DIRS[d][0];            final int dy = DIRS[d][1];            ans = Math.max(ans, 1 + dfs(grid, i + dx, j + dy, /*turned=*/false, 2, d, mem));          }     return ans;  }   private static final int[][] DIRS = {{-1, 1}, {1, 1}, {1, -1}, {-1, -1}};   private int dfs(int[][] grid, int i, int j, boolean turned, int num, int dir,                  Integer[][][][][] mem) {    if (i < 0 || i == grid.length || j < 0 || j == grid[0].length)      return 0;    if (grid[i][j] != num)      return 0;     final int hashNum = Math.max(0, num - 1);    if (mem[i][j][turned ? 1 : 0][hashNum][dir] != null)      return mem[i][j][turned ? 1 : 0][hashNum][dir];     final int nextNum = num == 2 ? 0 : 2;    final int dx = DIRS[dir][0], dy = DIRS[dir][1];    int res = 1 + dfs(grid, i + dx, j + dy, turned, nextNum, dir, mem);     if (!turned) {      final int nextDir = (dir + 1) % 4;      final int nextDx = DIRS[nextDir][0], nextDy = DIRS[nextDir][1];      res = Math.max(res,                     1 + dfs(grid, i + nextDx, j + nextDy, /*turned=*/true, nextNum, nextDir, mem));    }     return mem[i][j][turned ? 1 : 0][hashNum][dir] = res;  }}

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