【LeetCode】113.Path Sum II(Medium)解題報告

【LeetCode】113.Path Sum II(Medium)解題報告

題目地址：https://leetcode.com/problems/path-sum-ii/description/
題目描述：

  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]
]

Solution:

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 先序遍歷
 */
class Solution {
    public List<List<Integer>> pathSum(TreeNode root, int sum) {
        List<List<Integer>> res = new ArrayList<>();
        if(root == null) return res;
        helper(res , new ArrayList<>(),root,sum);
        return res;
    }
    public static void helper(List<List<Integer>> res,List<Integer> list,TreeNode root,int sum){
        if(root == null) return;
        list.add(root.val);
        if(root.left == null && root.right == null){
            if(sum == root.val){
                res.add(new ArrayList<>(list));
            }
        }
        helper(res,list,root.left,sum-root.val);
        helper(res,list,root.right,sum-root.val);
        list.remove(list.size()-1);
    }
}

Date：2018年3月14日