题解 | #二叉树根节点到叶子节点和为指定值的路径#

dfs+回溯

import java.util.*;


class TreeNode {
  int val = 0;
  TreeNode left = null;
  TreeNode right = null;
}


public class Solution {
    /**
     *
     * @param root TreeNode类
     * @param sum int整型
     * @return int整型ArrayList<ArrayList<>>
     */
    private ArrayList<ArrayList<Integer>> result;
    private int target;

    void dfs(TreeNode root, ArrayList<Integer> path, int sum) {
        if (root == null) return;
        if (isLeaf(root)) {
            if (sum + root.val == target) {
                ArrayList<Integer> newPath = (ArrayList<Integer>) path.clone();
                newPath.add(root.val);
                result.add(newPath);
            }
            return;
        }
        path.add(root.val);
        if (root.left != null) {
            dfs(root.left, path, sum + root.val);
        }
        if (root.right != null) {
            dfs(root.right, path, sum + root.val);
        }
        path.remove(path.size()-1);
    }

    boolean isLeaf(TreeNode root) {
        if (root.right == null && root.left == null) {
            return true;
        }
        return false;
    }
    public ArrayList<ArrayList<Integer>> pathSum (TreeNode root, int sum) {
        result = new ArrayList<>();
        target = sum;
        dfs(root, new ArrayList<>(), 0);
        return result;
    }
}