[LeetCode] 113. Path Sum II ☆☆☆(二叉树所有路径和等于给定的数) Path Sum II leetcode java

LeetCode 二叉树路径问题 Path SUM（①②③）总结

Path Sum II leetcode java

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]
]

解析

除了要判断是否有这样的一个path sum，还需要把所有的都可能性结果都返回，所以就用传统的DFS递归解决子问题。

将当前节点root的值放入list中更新sum值，判断当前节点是否满足递归条件root.left == null && root.right == null&&sum == 0；

若满足，则将存有当前路径的list值存入最后的大list中

然后依次递归左子树和右子树

从存有当前路径的list中去除最后一个节点，这样便可以返回到了当前叶子节点的父节点

代码

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    List<List<Integer>> listAll = new ArrayList<>();
    List<Integer> list = new ArrayList<>();
    public List<List<Integer>> pathSum(TreeNode root, int sum) {
        if(root == null) 
            return listAll;  
        list.add(root.val);  
        sum -= root.val;  
        if(root.left == null && root.right == null && sum == 0)  
            listAll.add(new ArrayList<Integer>(list));  
        pathSum(root.left, sum);  
        pathSum(root.right, sum);  
        list.remove(list.size() - 1); 
        return listAll;  
    }
}

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public void pathSumHelper(TreeNode root, int sum, List<Integer> sumlist, List<List<Integer>> pathlist) {
        if (root == null)
            return;
        sumlist.add(root.val);
        sum = sum - root.val;
        if (root.left == null && root.right == null) {
            if (sum == 0) {
                pathlist.add(new ArrayList<Integer>(sumlist));
            }
        } else {
            if (root.left != null)
                pathSumHelper(root.left, sum, sumlist, pathlist);
            if (root.right != null)
                pathSumHelper(root.right, sum, sumlist, pathlist);
        }
        sumlist.remove(sumlist.size() - 1);
    }

    public List<List<Integer>> pathSum(TreeNode root, int sum) {
        List<List<Integer>> pathlist = new ArrayList<List<Integer>>();
        List<Integer> sumlist = new ArrayList<Integer>();
        pathSumHelper(root, sum, sumlist, pathlist);
        return pathlist;
    }
}