Leetcode 112 Solution

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    bool hasPathSum(TreeNode* root, int sum) {
        stack<TreeNode*> s;

        if (root == NULL)
            return false;

        s.push(root);

        int curr = root->val;
        TreeNode* prev = NULL;
        while (s.empty() == false)
        {
            auto p = s.top();

            if (prev == NULL || (prev != p->left && prev != p->right))
            {
                if (p->left)
                {
                    s.push(p->left);
                    curr += p->left->val;
                    prev = p;
                }
                else if (p->right)
                {
                    s.push(p->right);
                    curr += p->right->val;
                    prev = p;
                }
                else
                {
                    if (curr == sum)
                        return true;

                    prev = p;
                    curr -= p->val;
                    s.pop();
                }
            }
            else if (prev == p->left && p->right)
            {
                s.push(p->right);
                curr += p->right->val;
                prev = p;
            }
            else
            {
                prev = p;
                curr -= p->val;
                s.pop();
            }
        }

        return false;
    }
};