LeetCode OJ 257. Binary Tree Paths

Given a binary tree, return all root-to-leaf paths.

For example, given the following binary tree:

   1
 /   \
2     3
 \
  5

All root-to-leaf paths are:

["1->2->5", "1->3"]

Credits:
Special thanks to @jianchao.li.fighter for adding this problem and creating all test cases.

这个题目的难点在与分裂路径，我们在遍历到一个节点的时候需要把到达它的路径告诉它，如果这个节点是叶子节点，则把它加入路径然后添加到路径集合中。如果它不是叶子节点，则把它添加到路径中，并且继续遍历它的子节点。代码如下：

 /**
  * Definition for a binary tree node.
  * public class TreeNode {
  *     int val;
  *     TreeNode left;
  *     TreeNode right;
  *     TreeNode(int x) { val = x; }
  * }
  */
 public class Solution {
     public List<String> list = new ArrayList();

     public List<String> binaryTreePaths(TreeNode root) {
         if(root == null) return list;
         String s = "";
         path(root, s);
         return list;
     }

     public void path(TreeNode root, String p){
         String s = new String(p);
         if(root.left==null && root.right==null){//如果是叶子节点
             s = s + root.val;
             list.add(s);
         }
         else s = s + root.val + "->";
         if(root.left!=null)                     //如果左子树非空
             path(root.left, s);
         if(root.right!=null)                    //如果右子树非空
             path(root.right, s);
     }
 }