543. Diameter of Binary Tree

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    int max_diameter = 0; // must be here to be accesible from the two functions below 
    
    public int diameterOfBinaryTree(TreeNode root) {
   
      maxDepth(root);
      return max_diameter;
        
    }
    private int maxDepth(TreeNode root){
      if (root == null){
        return 0;
      }
      
      
      int left = maxDepth(root.left);
      int right = maxDepth(root.right);
      
      int current_diameter = left + right;
      
      max_diameter = Math.max(current_diameter, max_diameter);
      
      return Math.max(right, left) + 1;
    }
}


// when solving this problem, maxdepth is really just retuning the max depth from the very bottom to the root, and while doing this, we have a current diameter to calculate the current left max depth  + right max depth , and compare it with previous max_diameter ,