543. Diameter of Binary Tree

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

var diameterOfBinaryTree = function(root) {
     // 在dfs的过程中，用一个外部变量保存当前的最大左手臂+右手臂的和
    var maxDiameter = 0;
    
    var dfs = function(root){
        if(root === null) return 0;
        let leftDepth = dfs(root.left);
        let rightDepth = dfs(root.right);
        maxDiameter = leftDepth + rightDepth > maxDiameter ? leftDepth + rightDepth : maxDiameter;
        
        return Math.max(leftDepth, rightDepth) + 1;
    }

    dfs(root);
    return maxDiameter
};