LeetCode - Path Sum II

題目：

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]
]

思路：

註意必須到 leaf node

package tree;

import java.util.ArrayList;
import java.util.List;

public class PathSumII {
    
    public List<List<Integer>> pathSum(TreeNode root, int sum) {
        List<List<Integer>> res = new ArrayList<List<Integer>>();
        if (root == null) return res;
        List<Integer> record = new ArrayList<Integer>();
        pathSum(res, record, root, sum);
        return res;
    }
    
    private void pathSum(List<List<Integer>> res, List<Integer> record, TreeNode root, int sum) {
        if (root == null && sum != 0) return;
        if (root == null && sum == 0) {
            res.add(record);
        } else {
            if (root.left == null || root.right == null) {
                List<Integer> newRecord = new ArrayList<Integer>(record);
                newRecord.add(root.val);
                pathSum(res, newRecord, root.left == null ? root.right : root.left, sum - root.val);
            } else {
                List<Integer> newRecord1 = new ArrayList<Integer>(record);
                newRecord1.add(root.val);
                pathSum(res, newRecord1, root.left, sum - root.val);
                List<Integer> newRecord2 = new ArrayList<Integer>(record);
                newRecord2.add(root.val);
                pathSum(res, newRecord2, root.right, sum - root.val);
            }
        }
    }
    
}