/**

 * Definition for a binary tree node.

 * public class TreeNode {

 *     int val;

 *     TreeNode left;

 *     TreeNode right;

 *     TreeNode() {}

 *     TreeNode(int val) { this.val = val; }

 *     TreeNode(int val, TreeNode left, TreeNode right) {

 *         this.val = val;

 *         this.left = left;

 *         this.right = right;

 *     }

 * }

 */

class Solution {

    boolean flag = false;

    public boolean hasPathSum(TreeNode root, int targetSum) {

        if(root == null) {

            return false;

        }

        dfs(root, targetSum);

        return flag;

    }

    public void dfs(TreeNode root, int num) {

        if(root == null) {

            return;

        }

        if(root.left == null && root.right == null)

            //注意，当值为root的值时，说明加上root就等于目标值

            if(num == root.val) {

                flag = true;

            }

        }

        dfs(root.left, num - root.val);

        dfs(root.right, num - root.val);

    }

}