LeetCode 124. Binary Tree Maximum Path Sum
A path in a binary tree is a sequence of nodes where each pair of adjacent nodes in the sequence has an edge connecting them. A node can only appear in the sequence at most once. Note that the path does not need to pass through the root.
The path sum of a path is the sum of the node’s values in the path.
Given the root of a binary tree, return the maximum path sum of any non-empty path.
Example 1:
Input: root = [1,2,3]
Output: 6
Explanation: The optimal path is 2 -> 1 -> 3 with a path sum of 2 + 1 + 3 = 6.Example 2:
Input: root = [-10,9,20,null,null,15,7]
Output: 42
Explanation: The optimal path is 15 -> 20 -> 7 with a path sum of 15 + 20 + 7 = 42.method
- 空节点返回0
- 非空节点可以累计其左节点和右节点，前提是左右节点都大于0
- 返回到上一层只能选择左节点或者右节点
int res = INT_MIN;
int dfs(TreeNode* root) {
if (!root) return 0;
int leftVal = max(dfs(root->left), 0);
int rightVal = max(dfs(root->right), 0);
int curVal = root->val + leftVal + rightVal;
res = max(res, curVal);
return root->val + max(leftVal, rightVal); // 返回只能选择其中一条
}
int maxPathSum(TreeNode* root) {
dfs(root);
return res;
}
