leetcode-112. Path Sum

leetcode-112. Path Sum

题目：

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.

典型的DFS。
迭代的终止条件：当前节点的值等于剩余的值，并且该节点为叶子节点
迭代过程：左右子节点结果相或

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public boolean hasPathSum(TreeNode root, int sum) {
        List<TreeNode> list = new ArrayList<TreeNode>();
        if(root==null) return false;
        return helper(list,root,sum);
    }

    private boolean helper(List<TreeNode> list ,TreeNode root, int sum){
        if(root == null) return false;
        if(sum==root.val && root.left == null && root.right == null) return true;

        list.add(root);
        return helper(list,root.left,sum-root.val) || helper(list,root.right,sum-root.val);
    }
}