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：
递归求解，只是要保存当前的结果，并且每次递归出来后要恢复递归前的结果，每当递归到叶子节点时就把当前结果保存下来。
/**
* Definition for a binary tree node.
* function TreeNode(val) {
* this.val = val;
* this.left = this.right = null;
* }
*/
/**
* @param {TreeNode} root
* @param {number} sum
* @return {number[][]}
*/
var pathSum = function(root, sum) {
var path=[],res=[];
if(root==null){
return [];
} path.push(root.val);
getPath(root,sum,path,res);
return res;
}; function getPath(root,sum,path,res){
path=path.concat();
if(root.left==null&&root.right==null&&root.val==sum){
res.push(path);
return;
}
if(root.left){
path.push(root.left.val);
getPath(root.left,sum-root.val,path,res);
path.pop();
}
if(root.right){
path.push(root.right.val);
getPath(root.right,sum-root.val,path,res);
path.pop();
}
}