DiameterOfBinaryTree.java

/*
Given the root of a binary tree, return the length of the diameter of the tree.
The diameter of a binary tree is the length of the longest path between any two nodes in a tree. This path may or may not pass through the root.
The length of a path between two nodes is represented by the number of edges between them.

Example 1:
Input: root = [1,2,3,4,5]
Output: 3
Explanation: 3 is the length of the path [4,2,1,3] or [5,2,1,3].
 */
import java.util.ArrayList;
import java.util.LinkedList;
import java.util.List;
import java.util.Queue;

public class DiameterOfBinaryTree {
    public static void main(String[] args) {
        TreeNode root=new TreeNode(1);
        root.left=new TreeNode(2);
        root.right=new TreeNode(3);
        root.left.left=new TreeNode(4);
        int ans = diameterOfBinaryTree(root);
        System.out.println(ans);
    }
    public static int diameterOfBinaryTree(TreeNode root) {
        List<Integer> result = new ArrayList<>();
        if (root == null) {
            return 0;
        }
        Queue<TreeNode> queue = new LinkedList<>();
        queue.offer(root);

        while (!queue.isEmpty()) {
            int levelSize = queue.size();
            for (int i = 0; i < levelSize; i++) {
                TreeNode cur = queue.poll();

                int leftDepth = maxDepth(cur.left);
                int rightDepth = maxDepth(cur.right);
                result.add(leftDepth + rightDepth);

                if (cur.left != null) {
                    queue.offer(cur.left);
                }
                if (cur.right != null) {
                    queue.offer(cur.right);
                }
            }
        }

        int max = Integer.MIN_VALUE;
        for (int num : result) {
            if (num > max) {
                max = num;
            }
        }
        return max;
    }

    public static int maxDepth(TreeNode root) {
        if (root == null) {
            return 0;
        }
        return 1 + Math.max(maxDepth(root.left), maxDepth(root.right));
    }
}