/**

• Definition for a binary tree node.
• public class TreeNode {

• int val;
  

• TreeNode left;
  

• TreeNode right;
  

• TreeNode() {}
  

• TreeNode(int val) { this.val = val; }
  

• TreeNode(int val, TreeNode left, TreeNode right) {
  

•     this.val = val;
  

•     this.left = left;
  

•     this.right = right;
  

• }
  

• }
  */
  class Solution {
  int res = 0;
  public int diameterOfBinaryTree(TreeNode root) {
  solve(root);
  return res;
  }

int solve(TreeNode root){
if(root == null){
return 0;
}
int left = solve(root.left);
int right = solve(root.right);
res = Math.max(left + right, res);
return Math.max(left, right) + 1;
}

}

时间：O（n）
空间：O（n）