129. Sum Root to Leaf Numbers

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
/**
Recursive DFS Solution.
Time Complexity = O(n). All nodes will be visited once.
Space Complexity = O(n). Recursion stack can grow up to n size if the tree is skewed.
*/
class Solution {
    public int sumNumbers(TreeNode root) {
        if (root == null) {
            return 0;
        }
        return sumNumbersHelper(root, 0);
    }

    public int sumNumbersHelper(TreeNode node, int currVal) {
        if (node == null) {
            return 0;
        }
        currVal = currVal * 10 + node.val;
        if (node.left == null && node.right == null) {
            return currVal;
        }
        return sumNumbersHelper(node.left, currVal) + sumNumbersHelper(node.right, currVal);
    }
}

/**
Recursive Post Order Traversal
Time Complexity: O(n) -> Each node is visited twice.
Space Complexity: O(n) -> Stack can grow upto all nodes if the tree is skewed.
*/
class Solution {
    public int sumNumbers(TreeNode root) {
        if (root == null) {
            return 0;
        }

        Stack<TreeNode> stack = new Stack<>();
        int result = 0;
        int num = 0;
        TreeNode cur = root;
        TreeNode pre = null;

        while (cur != null || !stack.isEmpty()) {
            while (cur != null) {
                stack.push(cur);
                num = num * 10 + cur.val;
                cur = cur.left;
            }
            cur = stack.peek();
            if (cur.right != null && pre != cur.right) {
                cur = cur.right;
                continue;
            }
            if (cur.left == null & cur.right == null) {
                result += num;
            }
            pre = stack.pop();
            num /= 10;
            cur = null;
        }
        return result;
    }
}

/**
DFS + Stack
Time Complexity: O(n)
Space Complexity: O(n)
*/
import javafx.util.Pair;
class Solution {
    public int sumNumbers(TreeNode root) {
        if (root == null) {
            return 0;
        }
        Stack<Pair<TreeNode, Integer>> stack = new Stack<>();
        stack.push(new Pair<TreeNode, Integer>(root, root.val));
        int result = 0;

        while (!stack.isEmpty()) {
            Pair<TreeNode, Integer> pair = stack.pop();
            if (pair.getKey().left == null && pair.getKey().right == null) {
                result += pair.getValue();
            }
            if (pair.getKey().left != null) {
                TreeNode left = pair.getKey().left;
                int leftVal = pair.getValue() * 10 + left.val;
                stack.push(new Pair<TreeNode, Integer>(left, leftVal));
            }
            if (pair.getKey().right != null) {
                TreeNode right = pair.getKey().right;
                int rightVal = pair.getValue() * 10 + right.val;
                stack.push(new Pair<TreeNode, Integer>(right, rightVal));
            }
        }
        return result;
    }
}