LeetCode 113. Path Sum II（路径和）

原题网址：https://leetcode.com/problems/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.
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]
]

方法：深度优先搜索。

/**
* Definition for a binary tree node.
* public class TreeNode {
*     int val;
*     TreeNode left;
*     TreeNode right;
*     TreeNode(int x) { val = x; }
* }
*/
public class Solution {
private List<List<Integer>> paths = new ArrayList<>();
private void find(TreeNode root, List<Integer> values, int currentSum, int targetSum) {
if (root == null) return;
values.add(root.val);
currentSum += root.val;
if (root.left != null && root.right != null) {
find(root.left, values, currentSum, targetSum);
find(root.right, values, currentSum, targetSum);
} else if (root.left != null) {
find(root.left, values, currentSum, targetSum);
} else if (root.right != null) {
find(root.right, values, currentSum, targetSum);
} else if (currentSum == targetSum) {
List<Integer> path = new ArrayList<>(values.size());
path.addAll(values);
paths.add(path);
}
values.remove(values.size()-1);
}
public List<List<Integer>> pathSum(TreeNode root, int sum) {
find(root, new ArrayList<>(), 0, sum);
return paths;
}
}

另一种实现：

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