543. 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.

class Solution {
    public int diameterOfBinaryTree(TreeNode root) {
        int[] res = new int[1];
        help(root, res);
        return res[0];
    }
    
    public int help(TreeNode root, int[] res) {
        if(root == null) return 0;
        
        int lh = help(root.left, res);
        int rh = help(root.right, res);
        res[0] = Math.max(res[0], lh + rh);
        return Math.max(lh, rh) + 1;
    }
}

https://www.youtube.com/watch?v=3vZV-6qPDmE 看了一姐的视频恍然大悟。

res[0], 以这个点为root的左子树路径长度+右子树路径长度

return:  如果父亲节点往当前节点走，当前节点最多能提供的路径长度。也就是说，返回当前节点的左子树和右子树路径长度中大的一方（因为要最长，所以不能返回lh+rh而只能返回一边），然后+1，这个+1是指当前节点走到它的父亲节点的长度是1

整体来说，每个节点遍历完左右子树后，都会尝试更新res by comparing current res and (lh + rh), and return a number represent the longest path the left subtree or rightsubtree can give plus it goes back to root.