[LeetCode] Path Sum II

Path Sum II

Given a binary tree and a sum, find all root-to-leaf paths where each path's sum equals the given sum.

For example:
Given the below binary tree and sum = 22,

              5
             / \
            4   8
           /   / \
          11  13  4
         /  \    / \
        7    2  5   1

return

[
   [5,4,11,2],
   [5,8,4,5]
]

Solution: Backtracking is the most space-efficient way to solve this.
Code in Java: Non-backtracking.

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    private ArrayList<List<Integer>> result;
    
    void pathSum(
        TreeNode root, 
        int sum,
        ArrayList<Integer> path
    ) {
        if (root == null) { return; }
        path.add(root.val);
        sum -= root.val;
        if (root.left == null && root.right == null && sum == 0) {
            result.add(new ArrayList<Integer>(path));
        }
        pathSum(root.left, sum, new ArrayList(path));
        pathSum(root.right, sum, new ArrayList(path));
    }
    
    public List<List<Integer>> pathSum(TreeNode root, int sum) {
        result = new ArrayList<List<Integer>>();
        pathSum(root, sum, new ArrayList<Integer>());
        return result;
    }
}

Code in Java: Backtracking.

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    private ArrayList<List<Integer>> result;
    ArrayList<Integer> path;
    
    void pathSumBackTracking(
        TreeNode root, 
        int sum
    ) {
        if (root == null) { return; }
        path.add(root.val);
        sum -= root.val;
        if (root.left == null && root.right == null && sum == 0) {
            result.add(new ArrayList<Integer>(path));
        }
        pathSumBackTracking(root.left, sum);
        pathSumBackTracking(root.right, sum);
        path.remove(path.size() - 1);
    }
    
    public List<List<Integer>> pathSum(TreeNode root, int sum) {
        result = new ArrayList<List<Integer>>();
        path = new ArrayList<Integer>();
        pathSumBackTracking(root, sum);
        return result;
    }
}