LeetCode 112. Path Sum

Problem

LeetCode 112. Path Sum

1. 题目简述

给出一棵二叉树和一个sum值，根节点为root，求从root节点到叶子节点是否存在一条路径其节点之和为sum。例如：

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.

2. 算法思路

DFS

很简单的DFS的思路，中序遍历的思路，记录从根节点到当前节点的和，到达根节点时做一次判断。

3. 解法

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    public boolean hasPathSum(TreeNode root, int sum) {
        if (root == null) {
            return false;
        }
        if (root.left == null && root.right == null && sum == root.val) {
            return true;
        }

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