题解 | #牛群特殊路径的数量#

import java.util.*;

/*
 * public class TreeNode {
 *   int val = 0;
 *   TreeNode left = null;
 *   TreeNode right = null;
 *   public TreeNode(int val) {
 *     this.val = val;
 *   }
 * }
 */

public class Solution {
    /**
     * 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
     *
     * 
     * @param root TreeNode类 
     * @param sum int整型 
     * @return int整型
     */
    public int pathSum (TreeNode root, int sum) {
        // write code here
        if(root == null){
            return 0;
        }
        int result = dfs(root,sum);
        result += pathSum(root.left,sum);
        result += pathSum(root.right,sum);
        return result;
    }

    public int dfs(TreeNode node,int sum){
        if(node == null || node.val > sum){
            return 0;
        }

        int result = 0;
        if(node.val == sum){
            result++;
        }

        result += dfs(node.left,sum - node.val);
        result += dfs(node.right,sum -node.val);
        return result;
    }
}