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);
    }
}