30:Path Sum

/* 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);
  }