/* Given a binary tree and a sum, determine if the tree has a root-to-leaf path such that adding up all the values along the path equals the given sum.

For example:

Given the below binary tree and sum = 22,

              5

             / \

            4   8

           /   / \

          11  13  4

         /  \      \

        7    2      1

return true, as there exist a root-to-leaf path 5->4->11->2 which sum is 22.

     * */

 

使用常见的递归算法：

public boolean hasPathSum_standard(TreeNode root, int sum) {

      if(root == null) return false;

      if(root.left == null && root.right == null && sum - root.val == 0)

          return true;

      return hasPathSum(root.left, sum - root.val) || hasPathSum(root.right, sum - root.val);

  }