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.

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Seen this question in a real interview before?   

Yes

 

/** * 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 max = 0;    int diameterOfBinaryTree(TreeNode* root) {        __diameterOfBinaryTree(root);        return max;    }        int __diameterOfBinaryTree(TreeNode* root) {        if(!root){             return 0;        }else{            int leftDepth = __diameterOfBinaryTree(root->left);            int rightDepth = __diameterOfBinaryTree(root->right);            if(leftDepth + rightDepth > max){                max = leftDepth + rightDepth;            }             return max(leftDepth +1, rightDepth + 1);        }    }};

深度搜索的思想，利用一个全局变量来存储当前的最大值。