113.Path Sum II

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Question

Given a binary tree and a sum, find all root-to-leaf paths where each path’s sum equals the given sum.

Note: A leaf is a node with no children.

Example:

Given the below binary tree and sum = 22,

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2
3
4
5
6
7
      5
     / \
    4   8
   /   / \
  11  13  4
 /  \    / \
7    2  5   1

Return:

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[
   [5,4,11,2],
   [5,8,4,5]
]

Answer

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/**
 * 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>> res = new ArrayList<>() ;
    
    
    public List<List<Integer>> pathSum(TreeNode root, int sum) {
        List<Integer> list = new ArrayList<>();
        pathSum2(root, sum, list);
        return res;
        
    }
    
    public void pathSum2(TreeNode root, int sum, List<Integer> resList){
        if(root == null)
            return;
        if(root.left == null && root.right == null && root.val == sum){ // leaf
            resList.add(root.val);
            res.add(resList);
            return;
        }
        resList.add(root.val);
        List<Integer> resnext = new ArrayList(resList);
        pathSum2(root.left, sum-root.val, resnext); 
        resnext = new ArrayList(resList);
        pathSum2(root.right, sum-root.val, resnext);
    }
}

Time complexity: $O(n)$

Space complexity: $O(n)$