LeetCode HOT100 - 二叉树的直径

树 dp

左子树的（链）最大长度，右子树的最大长度

直径就是左加右 + 1

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
public:
    int diameterOfBinaryTree(TreeNode* root) {
        int ans = 0;
        auto dfs = [&](this auto&& self, TreeNode* u) -> int {
            if (!u) {
                return 0;
            }
            int l = self(u->left);
            int r = self(u->right);
            ans = max(l + r + 1, ans);
            // cout << u->val << ' ';
            // cout << l << ' ' << r << '\n';
            return max(l, r) + 1;
        };
        dfs(root);
        return ans - 1;
    }
};