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package Algorithms.tree;

import java.util.ArrayList;

import java.util.List;

/**

* Definition for binary tree

* public class TreeNode {

* int val;

* TreeNode left;

* TreeNode right;

* TreeNode(int x) { val = x; }

* }

*/

public class PathSum2 {

public List<List<Integer>> pathSum(TreeNode root, int sum) {

List<List<Integer>> ret = new ArrayList<List<Integer>>();

ArrayList<Integer> path = new ArrayList<Integer>();

pathSumHelp(root, sum, path, ret);

return ret;

}

public void pathSumHelp(TreeNode root, int sum, ArrayList<Integer> path, List<List<Integer>> ret) {

if (root == null) {

return;

}

path.add(root.val);

if (root.left == null

&& root.right == null

&& root.val == sum) {

ret.add(new ArrayList<Integer>(path));

} else {

// 继续递归

pathSumHelp(root.left, sum - root.val, path, ret);

pathSumHelp(root.right, sum - root.val, path, ret);

}

// 注意，递归和回溯的特点就是 递归不可以改变path的值。也就是说，你返回时，这个path不能被改变

// 所以在这里要执行remove操作。

path.remove(path.size() - 1);

}

}