题目如下：
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.
解决思路是：求出每一个点左右的最大深度，然后就可以计算出以该节点作为根节点的最大。
/**
* 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 result = 0;
int diameterOfBinaryTree(TreeNode* root) {
maxDeep(root);
return result;
}
int maxDeep(TreeNode* root) {
if(root == NULL) {
return -1;
}
else if(root->left == NULL && root->right == NULL) {
return 0;
}
int leftDepth = maxDeep(root->left);
int rightDepth = maxDeep(root->right);
result = max(leftDepth + rightDepth + 2, result);
return max(leftDepth, rightDepth) + 1;
}
};