LeetCode 39 Path Sum II

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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]]

分析：

看到二叉树，想递归，

看到find all，想回溯，继而想到 DFS。

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

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