Problem(root to leaf)

Given a binary tree, find all paths that sum of the nodes in the path equals to a given number target.

A valid path is from root node to any of the leaf nodes.

Example
Given a binary tree, and target = 5:

     1
    / \
   2   4
  / \
 2   3
return

[
  [1, 2, 2],
  [1, 4]
]

思路(Traverse)

1. backtrack

    public List<List<Integer>> binaryTreePathSum(TreeNode root, int target) {
        List<List<Integer>> result = new ArrayList<>();
        helper(root, target, result, 0, new ArrayList<Integer>());
        return result;
    }

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

        sum += root.val;
        path.add(root.val);
        if (root.left == null && root.right == null) {
            if (target == sum) {
                result.add(new ArrayList<Integer>(path));
            }
            // backtracking
            // because you have a return statement here
            // path.remove(path.size() - 1);
            // return;
        }

        helper(root.left, target, result, sum, path);
        helper(root.right, target, result, sum, path);

        // backtracking
        sum -= root.val;
        path.remove(path.size() - 1);
    }