543. Diameter of Binary Tree

postorder O(n) 跟124. Binary Tree Maximum Path Sum思路基本一致

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    int diameterOfBinaryTree(TreeNode* root) {
        dfs(root);
        return res;
    }

    int dfs(TreeNode *root) { // 返回以root为根的最长链有几个结点
        if (!root) return 0;
        int l = dfs(root->left), r = dfs(root->right);
        res = max(res, l + r); // 人家问的是边不是点，不要加1
        return max(l, r) + 1;
    }

    int res = 0;
};