*LeetCode-Path Sum II

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和以前做的backtracking很像但是注意要删除这次add的node！！否则左边recurse加上了left 到了右边却没有remove left就继续了

public class Solution {    public List<List<Integer>> pathSum(TreeNode root, int sum) {        List<List<Integer>> res = new ArrayList<List<Integer>> ();        List<Integer> list = new ArrayList<Integer> ();        helper ( res, list, root, sum );        return res;    }    public void helper ( List<List<Integer>> res, List<Integer> list, TreeNode root, int sum ){        if ( root == null )            return;        list.add ( root.val );        if ( root.val == sum && root.left == null && root.right == null ){            res.add ( new LinkedList (list) );            list.remove( list.size() - 1 );            return;        }        helper ( res, list, root.left, sum - root.val );        helper ( res, list, root.right, sum - root.val );        list.remove( list.size() - 1 );    }}

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