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);
}
}