LeetCode - Path Sum II

题目：

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：

注意必须到 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);
}
}
}

}