543. Diameter of Binary Tree (Easy)

https://leetcode.com/problems/diameter-of-binary-tree/

Given a binary tree, you need to compute 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.

Example:
Given a binary tree

          1
         / \
        2   3
       / \     
      4   5    

Return 3, which is the length of the path [4,2,1,3] or [5,2,1,3].

Note: The length of path between two nodes is represented by the number of edges between them.

Solutions

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public int diameterOfBinaryTree(TreeNode root) {
        // track keeps the depth of longest children branches for each node
        Map<TreeNode, Integer> track = new HashMap<>();

        recurse(root, 1, track);

        int ans = 0;
        for (TreeNode node : track.keySet()) {
            ans = Math.max(track.get(node), ans);
        }

        // depth == edge, so return depth directly
        return ans;
    }

    private int recurse(TreeNode root, int depth, Map<TreeNode, Integer> track) {
        // root is null means for current path, the longest depth is depth - 1
        if (root == null) {
            return depth - 1;
        }

        // depth of left branch
        int ld = recurse(root.left, depth + 1, track);

        // depth of right branch
        int rd = recurse(root.right, depth + 1, track);

        // For each node, the diameter is the sum of left + right branches.
        track.put(root, ld - depth + rd - depth);

        // return the max length branch as the longest child branch of parent branch
        return Math.max(ld, rd);
    }
}

Incorrect Solutions

References

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