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# 543. Diameter of Binary Tree

## 1.問題

* 計算diameter: 任意兩node間最長的path

![](/files/-LSCyArerjOb_NpomPe0)

## 2.想法

* 參考104. Maximum Depth of Binary Tree
* 回傳值為 max(left path, right path) +1
* res = max(res, left path + right path)

## 3.程式碼

```
/**
 * 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 == NULL){
            return 0;
        } 
        int res = 0;
        getHeight(root, res);
        return res;
    }
    int getHeight(TreeNode* root, int& res){
        if(root == NULL) {
            return 0;
        }

        int leftlength= getHeight(root->left,res);
        int rightlength = getHeight(root->right,res);
        //取在該點為root時的leftlength + rightlength的最大值
        res = max(res, leftlength + rightlength);
        //回傳max(左邊, 右邊)+1
        return max(leftlength, rightlength) + 1;

    }
};
```