543. Diameter of Binary Tree

题目描述【Leetcode】

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.

这个就是求两个节点之间最长路径，用求深度的办法来做这道题

/** * Definition for a binary tree node. * struct TreeNode { *     int val; *     TreeNode *left; *     TreeNode *right; *     TreeNode(int x) : val(x), left(NULL), right(NULL) {} * }; */int dep(TreeNode*root){     if(root == NULL) return 0;     int l = dep(root->left);     int r = dep(root->right);     if(l > r) return l+1;     else return r+1;}class Solution {public:    int diameterOfBinaryTree(TreeNode* root) {        if(root == NULL) return 0;        int n = dep(root->left)+dep(root->right);        return max(n,max(diameterOfBinaryTree(root->left),diameterOfBinaryTree(root->right)));    }};

网上有一个运行时间更少的办法：

`class Solution {
public:

int helper(TreeNode* root, int& h) {    if (!root) {        h = 0;        return 0;    }    int lh, rh;    int ld = helper(root->left, lh);    int rd = helper(root->right, rh);

    h = 1 + max(lh, rh);    return max(lh + rh, max(ld, rd));}int diameterOfBinaryTree(TreeNode* root) {    int height;    return helper(root, height);}

};`