112. 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.

 

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链接：http://leetcode.com/problems/path-sum/ 

题解：

求二插树中是否存在路径使得节点和为目标值。当节点左右子节点都为空时才是leaf，所以使用DFS就可以了。有空要研究一下递归的空间复杂度以及Morris Travel.

Time Complexity- O(n)， Space Complexity - O(n)。

public class Solution {

    public boolean hasPathSum(TreeNode root, int sum) {

        if(root == null)

            return false;        

        sum -= root.val;

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

            return true;

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

    }

}

 

测试：