leetcode 124. Binary Tree Maximum Path Sum （DFS）

Given a non-empty binary tree, find the maximum path sum.

For this problem, a path is defined as any sequence of nodes from some starting node to any node in the tree along the parent-child connections. The path must contain at least one node and does not need to go through the root.

Example 1:

Input: [1,2,3]

   1
  / \
 2   3

Output: 6
Example 2:

Input: [-10,9,20,null,null,15,7]

-10
/
9 20
/
15 7

Output: 42

• 思路：DFS，这道题和 leetcode 543 树的直径 很像

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
public:
    int res;
    int maxPathSum(TreeNode* root) {
        res=INT_MIN;
        dfs(root);
        return res;
    }
    int dfs(TreeNode* rt){
        if(rt==NULL)return 0;
        int left=max(0,dfs(rt->left));
        int right=max(0,dfs(rt->right));
        res=max(res,left+right+rt->val);
        return max(left,right)+rt->val;
    }
};