Leetcode(112) Path Sum

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Description

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,

1
2
3
4
5
6
7
      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.

解法

递归地解这道题，注意空节点的处理。这里没有剪枝，因为值可能为负数

具体代码如下：

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class Solution {
    public boolean hasPathSum(TreeNode root, int sum) {
        return pathsum(0, sum, root);
    }

    Boolean pathsum(int now, int target, TreeNode node) {
        if (node == null)
            return false;
        now = now + node.val;
        if (node.left == null && node.right == null) {
            if (now == target)
                return true;
            else
                return false;
        }
        return pathsum(now, target, node.right) || pathsum(now, target, node.left);
    }
}