[leetcode]113. Path Sum II路径和(返回路径)

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,

      5
     / 
    4   8
   /   / 
  11  13  4
 /      / 
7    2  5   1

Return:

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

题意：

二叉树之和，返回所有和等于给定值的路径

思路：

要穷举所有路径。 backtracking。

思路类似two sum, 通过（sum - 当前值） 来check 叶子节点的值是否与一路作减法的sum值相等

代码：

class Solution {
      public List<List<Integer>> pathSum(TreeNode root, int sum) {
        List<List<Integer>> result = new ArrayList<>();
        ArrayList<Integer>  cur = new ArrayList<>(); // 中间结果
        helper(root, sum, cur, result);
        return result;
     }
    
    
    private static void helper(TreeNode root, int sum, ArrayList<Integer> cur,
                                List<List<Integer>> result) {
        if (root == null) return;

        cur.add(root.val);

        // leaf node 
        if (root.left == null && root.right == null) {
            if (sum == root.val)
             result.add(new ArrayList<>(cur));
        }
        
        helper(root.left, sum - root.val, cur, result);
        helper(root.right, sum - root.val, cur, result);

        cur.remove(cur.size() - 1);
    }
}