543. Diameter of Binary Tree

//可以任意节点开始left+right 所以dfs的时候要每个节点都计算一下
class Solution {
    int max = Integer.MIN_VALUE;
    public int diameterOfBinaryTree(TreeNode root) {
        if(root == null) return 0;
        dfs(root);
        return max;
    }
    
    public int dfs(TreeNode root){
        if(root == null) return 0;
        int left = dfs(root.left);
        int right = dfs(root.right);
        int maxNow = left + right;
        max = Math.max(maxNow, max);
        return Math.max(right, left) + 1;
    }
}