[leetcode] 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.

For 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.

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

 

思路：递归求解。

　　注意，val可能是负数。注意判断是叶子结点，需要判断 left 和 right 都是null

/**
 * 
 * be careful: the val might be negtive...
*/
public class Solution {
    public boolean hasPathSum(TreeNode root, int sum) {
        if (root == null)
            return false;

        if (sum == root.val && root.left == null && root.right == null)
            return true;

        return hasPathSum(root.left, sum - root.val) || hasPathSum(root.right, sum - root.val);

    }

    public static void main(String[] args) {
        TreeNode root = new TreeNode(-5);
        root.left = new TreeNode(4);
        root.left.left = new TreeNode(11);
        root.right = new TreeNode(8);
        root.right.right = new TreeNode(5);

        System.out.println(new Solution().hasPathSum(root, 10));
    }
}

 

 第二遍记录：

public class Solution {
    public boolean hasPathSum(TreeNode root, int sum) {
        if(root==null)
            return false;
        if(root.val==sum && root.left==null && root.right ==null)
            return true;
        return hasPathSum(root.left,sum-root.val)||hasPathSum(root.right,sum-root.val);
    }
}