[LeetCode] Problem 257 - Binary Tree Paths

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

Note

A leaf is a node with no children.

Example

Input:

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5
   1
 /   \
2     3
 \
  5

Output: [“1->2->5”, “1->3”]

Explanation: All root-to-leaf paths are: 1->2->5, 1->3

Code

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public class TreeNode {
    int val;
    TreeNode left;
    TreeNode right;
    TreeNode(int x) { val = x; }
}
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public List<String> binaryTreePaths(TreeNode root) {
    List<String> result = new ArrayList<>();

    if (root == null)
        return result;

    StringBuilder sb = new StringBuilder();
    helper(result, root, sb.append(root.val));
    return result;
}

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

    int length = sb.length();

    if (root.left != null) {
        helper(result, root.left, sb.append("->").append(root.left.val));
        sb.setLength(length);
    }
    
    if (root.right != null) {
        helper(result, root.right, sb.append("->").append(root.right.val));
        sb.setLength(length);
    }
}