Longest Univalue Path【數的最長路徑】

PROBLEM：

Given a binary tree, find the length of the longest path where each node in the path has the same value. This path may or may not pass through the root.

Note: The length of path between two nodes is represented by the number of edges between them.

Example 1:

Input:

              5
             / \
            4   5
           / \   \
          1   1   5

Output:

2

Example 2:

Input:

              1
             / \
            4   5
           / \   \
          4   4   5

Output:

2

Note: The given binary tree has not more than 10000 nodes. The height of the tree is not more than 1000.

SOLVE：

class Solution {
public:
    int longestUnivaluePath(TreeNode* root) {
        int lup = 0;
        if (root) dfs(root, lup);
        return lup;
    }
private:
    int dfs(TreeNode* node, int& lup) {
        //lup：以node爲根節點的樹中最長路徑
        int l = node->left ? dfs(node->left, lup) : 0;    //l：左子樹從根節點開始的最長路徑 //注意：不一定是左子樹的最長路徑 //lup：更新爲左子樹的最長路徑
        int r = node->right ? dfs(node->right, lup) : 0;    //r：右子樹從根節點開始的最長路徑 //注意：不一定是右子樹的最長路徑 //lup：更新爲左子樹及右子樹中的最長路徑
        int resl = node->left && node->left->val == node->val ? l + 1 : 0;
        int resr = node->right && node->right->val == node->val ? r + 1 : 0;
        lup = max(lup, resl + resr);    //lup：更新後爲以node爲根節點的樹中最長路徑 //注意：路徑是邊數，而不是節點數
        return max(resl, resr);    //從根節點開始的最長路徑
    }
};