LeetCode--Binary Tree Maximum Path Sum

递归，dfs

这里返回值是一个pair<int,int>，first表示两个子树的不经过当前root的最大pathsum；second表示两个子树的，以root->left和root->right为路径起始节点的路径的最大值

/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    int maxPathSum(TreeNode *root) {
        pair<int,int> res = dfs(root);
        return res.first;
    }
    pair<int,int> dfs(TreeNode *root)
    {
        if(root == NULL)
        {
            return make_pair(0,0);
        }
        pair<int,int> left = dfs(root->left);
        pair<int,int> right = dfs(root->right);
        int leftCrossMax = left.first;
        int rightCrossMax = right.first;
        
        int leftMax = left.second > 0?left.second:0;
        int rightMax = right.second > 0?right.second:0;
        
        int maxNoCross = INT_MIN;
        if(root->left != NULL)
        {
            maxNoCross = max(maxNoCross,leftCrossMax);
        }
        if(root->right != NULL)
        {
            maxNoCross = max(maxNoCross,rightCrossMax);
        }
        int maxCross = leftMax+rightMax+root->val;
        int maxSum = max(maxCross,maxNoCross);
        
        int child = max(leftMax,rightMax);
        int maxChild = child > 0?child+root->val:root->val;
        return make_pair(maxSum,maxChild);
    }
};