543. Diameter of Binary Tree

Q:

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.

A:

 利用C++ 的参数reference type, 来完成 多个 返回值

看helper(     , int & maxDepth)

/**
 * 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) {
        // use reference to get the modified value from other function
        int dummy=0;
        return helper(root, dummy);
    }
private:
    int helper(TreeNode* root, int & maxDepth) // return the diameter of the tree
    {
        if(!root)
        {
            maxDepth = 0;
            return 0;
        }
        int leftDepth, rightDepth;
        int maxLeft = helper(root->left, leftDepth);
        int maxRight = helper(root->right, rightDepth);
        maxDepth = max(leftDepth, rightDepth) + 1;
        return max(maxLeft , max( maxRight, leftDepth + rightDepth) );        
    }
};