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.

Note: A leaf is a node with no children.

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]
]

class Solution {
    public void backtrack(TreeNode root, int sum, List<List<Integer>> ans, ArrayList<Integer> temp, int currentSum) {
        if (root == null)
            return;
        if (root.left == null && root.right == null) {
            if (currentSum + root.val == sum) {
                temp.add(root.val);
                ans.add(new ArrayList<>(temp));
                temp.remove(temp.size() - 1);
            }
            return;
        }
        temp.add(root.val);
        backtrack(root.left, sum, ans, temp, currentSum + root.val);
        backtrack(root.right, sum, ans, temp, currentSum + root.val);
        temp.remove(temp.size() - 1);
    }

    public List<List<Integer>> pathSum(TreeNode root, int sum) {
        List<List<Integer>> ans = new ArrayList<>();
        backtrack(root, sum, ans, new ArrayList<Integer>(), 0);
        return ans;
    }
}