[itint5]树中最大路径和

http://www.itint5.com/oj/#13

要注意，一是空路径也可以，所以最小是0。然后要时刻注意路径顶多有两条子路径+根节点组成，所以更新全局最值时和返回上一级的值要注意分清。

#include <climits>

using namespace std;



int maxPathHelper(TreeNode *root, int &max) {

    if (root == NULL) {

        return 0;

    }

    int root_val = root->val; // root it self

    int max1 = 0;

    int max2 = 0;

    for (int i = 0; i < root->children.size(); i++) {

        int max_sub = maxPathHelper(root->children[i], max);

        if (max_sub > max1) {

            max2 = max1;

            max1 = max_sub;

        } else if (max_sub > max2) {

            max2 = max_sub;

        }

    }

    int local_max = root_val;

    int tmp = root_val + max1;

    if (tmp > local_max) local_max = tmp;

    if (local_max > max) max = local_max;

    tmp = root_val + max1 + max2;

    if (tmp > max) max = tmp;

    return local_max;

}



/*

树结点的定义(请不要在代码中定义该类型)

struct TreeNode {

    int val;

    vector<TreeNode*> children;  //该结点的儿子结点

 };

*/

int maxTreePathSum(TreeNode *root) {

    int max = 0;

    maxPathHelper(root, max);

    return max;

}