LeetCode - 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 thelongest 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.

给出一颗二叉树，问你最长的一条路有多少条边。

就是求一下每个结点的左右子树高，然后求和就是路径长。因为是从下向上遍历树，时间复杂度O(n)，空间复杂度O(1)

/**
* 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) {
if (!root) return 0;
int ans = 0;
solve(root, ans);
return ans;
}
int solve(TreeNode* root, int &ans) {
if (!root) return 0;
int le = solve(root->left, ans);
int ri = solve(root->right, ans);

int sum = le + ri;
ans = max(ans, sum);
return max(le, ri) + 1;
}
};