diameter-of-binary-tree.js

/**
 * Definition for a binary tree node.
 * function TreeNode(val, left, right) {
 *     this.val = (val===undefined ? 0 : val)
 *     this.left = (left===undefined ? null : left)
 *     this.right = (right===undefined ? null : right)
 * }
 */
/**
 * @param {TreeNode} root
 * @return {number}
 */

var diameterOfBinaryTree = function(root) {
    let diameter = 0;
    dfs(root);
    return diameter;

    function dfs(node) {
        if (!node) return 0;

        const left = dfs(node.left);
        const right = dfs(node.right);

        // update diameter at every node
        
        diameter = Math.max(diameter, left + right);

        // update the max path of the current node
        // 1 is the height of the node itself + longest path of children
        
        return 1 + Math.max(left, right);
   
    }     
};