543. Diameter of Binary Tree (Easy)

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.

Solution: 9ms

For every node, length of longest path which pass it = MaxDepth of its left subtree + MaxDepth of its right subtree.

/**
 * 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 {
    int res = 0;
    int helper(TreeNode* root) {
        if (!root) return -1;
        int l = helper(root->left)+1, r = helper(root->right)+1;
        res = max(l+r, res);
        return max(l,r);
    }
public:
    int diameterOfBinaryTree(TreeNode* root) {
        helper(root);
        return res;
    }
};