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Problem solution · Java

Binary Tree Cameras

Binary Tree Cameras: 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
26 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 Binary Tree Cameras, 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

  • 26 lines of Java from the credited upstream file 968.java.
  • The implementation visibly relies on sequence storage.
  • No explicit 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 codeBinary Tree Cameras · JavaJava
Use this to learn the idea, then write your own version.
class Solution {  public int minCameraCover(TreeNode root) {    int[] ans = dfs(root);    return Math.min(ans[1], ans[2]);  }   // 0 := all the nodes below the root are covered except the root  // 1 := all the nodes below and including the root are covered with no camera  // 2 := all nodes below and including the root are covered with a camera  private int[] dfs(TreeNode root) {    if (root == null)      return new int[] {0, 0, 1000};     int[] l = dfs(root.left);    int[] r = dfs(root.right);     final int s0 = l[1] + r[1];    final int s1 = Math.min(l[2] + Math.min(r[1], r[2]), //                            r[2] + Math.min(l[1], l[2]));    final int s2 = Math.min(l[0], Math.min(l[1], l[2])) + //                   Math.min(r[0], Math.min(r[1], r[2])) + 1;     return new int[] {s0, s1, s2};  }}

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