112.Path Sum

Tags: Easy Tree Depth-first Search

Links: https://leetcode.com/problems/path-sum/

---

Given a binary tree and a sum, determine if the tree has a root-to-leaf path such that adding up all the values along the path equals the given sum.

Note: A leaf is a node with no children.

Example:

Given the below binary tree and sum = 22,

      5
     / \
    4   8
   /   / \
  11  13  4
 /  \      \
7    2      1

return true, as there exist a root-to-leaf path 5->4->11->2 which sum is 22.

---

class Solution {
public:
    bool hasPathSum(TreeNode* root, int sum) {
        if (!root) return false;
        if (!root->left && !root->right && root->val == sum ) return true;
        return hasPathSum(root->left, sum - root->val) ||
            hasPathSum(root->right, sum - root->val);
    }
};

递归解法，这题只需要注意是要求从顶至底的和等于sum，而不能是中间结点总和等于sum。

class Solution {
public:
    bool hasPathSum(TreeNode* root, int sum) {
        if (!root) return false;

        stack<TreeNode*> s;
        s.push(root);
        while (!s.empty()) {
            TreeNode* tmp = s.top(); s.pop();
            if (!tmp -> left && !tmp -> right && tmp -> val == sum) return true;

            if (tmp -> left) {
                tmp -> left -> val += tmp -> val;
                s.push(tmp -> left);
            }
            if (tmp -> right) {
                tmp -> right -> val += tmp -> val;
                s.push(tmp -> right);
            }
        }

        return false;
    }
};

迭代解法，先序遍历，利用辅助栈。