543. Diameter of Binary Tree

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.

My Solutions:

对ans来说，取的是ans和left+right中的最大值，因为一条diameter可以加上左边和右边的路径。但return的只能是max（left，right）+1，因为一个node只能通过一条路径

class Solution {
    
    int ans;
    
    public int diameterOfBinaryTree(TreeNode root) {
        if (root == null) return 0;
        di(root);
        return ans;
    }
    
    public int di(TreeNode root) {
        if (root == null) return 0;
        int left = di(root.left), right = di(root.right);
        ans = Math.max(ans, left + right);
        return Math.max(left, right) + 1;
    }
}