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.

int helper(TreeNode*& node, unordered_map<TreeNode*, int>& memo, int& res) {
    if (!node) return 0;
    if (memo.count(node)) return memo[node];
    int left = helper(node->left, memo, res), right = helper(node->right, memo, res);
    res = max(res, left + right);
    memo[node] = max(left, right) + 1;
    return memo[node];
}
int diameterOfBinaryTree(TreeNode* root) { // time: O(n); space: O(n)
    int res = 0;
    unordered_map<TreeNode*, int> memo;
    helper(root, memo, res);
    return res;
}