113. 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]
]

time: O(n), space: O(height)

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