112. 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 path5->4->11->2which sum is 22.

思路：递归DFS

public class Solution {
    public boolean hasPathSum(TreeNode root, int sum) {
        if (root==null) return false;

        if (root.left==null && root.right==null){
            return root.val == sum;
        }

        if (hasPathSum(root.left, sum-root.val)){
            return true;
        } 

        if (hasPathSum(root.right, sum-root.val)){
            return true;
        }

        return false;
    }
}