LeetCode - Path Sum

题目：

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.

思路：

仍是递归，但是得注意(1) 必须是到 leaf node (2)Null结点的判断

package tree;

public class PathSum {

    boolean isFirstRoot = true;

    public boolean hasPathSum(TreeNode root, int sum) {
        if (isFirstRoot && root == null) return false;
        isFirstRoot = false;
        if (root == null && sum == 0) return true;
        if (root == null && sum != 0) return false;
        if (root.left == null || root.right == null) return root.left == null ? hasPathSum(root.right, sum - root.val) : hasPathSum(root.left, sum - root.val);
        return hasPathSum(root.left, sum - root.val) || hasPathSum(root.right, sum - root.val);
    }

}