Binary Tree Paths 解答

Question

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"]

Solution -- Recursive

We can not apply inorder traversal here because it only show path from root to leaf node as required. Therefore, we use recursive way.

We consider two situation:

1. Current node is leaf. Add string to result.

2. Current node has children. Go on.

 /**
  * 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<String>();
         if (root == null)
             return result;
         dfs(root, result, new StringBuilder());
         return result;
     }

     private void dfs(TreeNode root, List<String> result, StringBuilder current) {
         if (root == null)
             return;
         if (root.left == null && root.right == null) {
             current.append(root.val);
             result.add(current.toString());
             return;
         }

         current.append(root.val);
         current.append("->");

         StringBuilder tmp1 = new StringBuilder(current);
         StringBuilder tmp2 = new StringBuilder(current);
         if (root.left != null)
             dfs(root.left, result, tmp1);
         if (root.right != null)
             dfs(root.right, result, tmp2);
     }
 }