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"]

Tips:

简单的DFS，不需要backtracking

Code：

recursion：

/**
 * 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> binaryTreePaths(TreeNode root) {
        List<String> result = new ArrayList<>();
        if (root == null) {
            return result;
        }
        dfs(root, result, "");
        return result;
    }
    private void dfs (TreeNode root, List<String> result, String s) {
        if(root.left == null && root.right==null){
            result.add(s+String.valueOf(root.val));
        }
        if(root.left != null) {
            dfs(root.left, result, s+String.valueOf(root.val)+"->");
        }
        if(root.right !=null) {
            dfs(root.right, result, s+String.valueOf(root.val)+"->");            
        }
    }
}

stack：

public class Solution {
    public List<String> binaryTreePaths(TreeNode root) {

        List<String> res = new ArrayList<String>();
        Map<TreeNode, String> nodeMap = new HashMap<TreeNode, String>();

        if (null == root) return res;

        Stack<TreeNode> stack = new Stack<TreeNode>();
        stack.push(root);
        nodeMap.put(root, String.valueOf(root.val));

        while(!stack.isEmpty()) {
            TreeNode node = stack.pop();

            if ((null == node.left) && (null == node.right)) {
                res.add(nodeMap.get(node));
            } 
            if (null != node.left) {
                stack.push(node.left);
                nodeMap.put(node.left, nodeMap.get(node)+"->" + String.valueOf(node.left.val));
            }
            if (null != node.right) {
                stack.push(node.right);
                nodeMap.put(node.right, nodeMap.get(node)+"->" + String.valueOf(node.right.val));
            }
            nodeMap.remove(node);

        }
        return res;
    }
}