Binary Tree Paths 480

Question

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

Example

Given the following binary tree:

1 / \ 2 3 \ 5

All root-to-leaf paths are:

[ "1->2->5",

"1->3" ]

Solution

Divide and Conquer.

九章给的答案是recursion.

代码如下：

D&C:

/**
 * Definition of TreeNode:
 * public class TreeNode {
 *     public int val;
 *     public TreeNode left, right;
 *     public TreeNode(int val) {
 *         this.val = val;
 *         this.left = this.right = null;
 *     }
 * }
 */
public class Solution {
    /**
     * @param root the root of the binary tree
     * @return all root-to-leaf paths
     */
    public List<String> binaryTreePaths(TreeNode root) {
        // Write your code here
        List<String> result = new ArrayList<String>();

        if(root == null){
            return result;
        }

        if(root.left == null && root.right == null){
            result.add(String.valueOf(root.val));
        }

        List<String> left = binaryTreePaths(root.left);
        List<String> right = binaryTreePaths(root.right);


        if(left.size() != 0){
            for(String s : left){
                StringBuilder sb = new StringBuilder();
                sb.append(root.val);
                sb.append("->");
                sb.append(s);
                result.add(sb.toString());
            }
        }

        if(right.size() != 0){
            for(String s : right){
                StringBuilder sb = new StringBuilder();
                sb.append(root.val);
                sb.append("->");
                sb.append(s);
                result.add(sb.toString());
            }
        }

        return result;
    }
}

Recursion:

public class Solution {
    /**
     * @param root the root of the binary tree
     * @return all root-to-leaf paths
     */
    public List<String> binaryTreePaths(TreeNode root) {
        List<String> result = new ArrayList<String>();
        if (root == null) {
            return result;
        }
        helper(root, String.valueOf(root.val), result);
        return result;
    }

    private void helper(TreeNode root, String path, List<String> result) {
        if (root == null) {
            return;
        }

        if (root.left == null && root.right == null) {
            result.add(path);
            return;
        }

        if (root.left != null) {
            helper(root.left, path + "->" + String.valueOf(root.left.val), result);
        }

        if (root.right != null) {
            helper(root.right, path + "->" + String.valueOf(root.right.val), result);
        }
    }
}