112. Path Sum. (Easy)

Given the root of a binary tree and an integer targetSum, return true if the tree has a root-to-leaf path such that adding up all the values along the path equals targetSum.

A leaf is a node with no children.

 

Example 1:

Input: root = [5,4,8,11,null,13,4,7,2,null,null,null,1], targetSum = 22
Output: true
Explanation: The root-to-leaf path with the target sum is shown.

Example 2:

Input: root = [1,2,3], targetSum = 5
Output: false
Explanation: There are two root-to-leaf paths in the tree:
(1 --> 2): The sum is 3.
(1 --> 3): The sum is 4.
There is no root-to-leaf path with sum = 5.

Example 3:

Input: root = [], targetSum = 0
Output: false
Explanation: Since the tree is empty, there are no root-to-leaf paths.

 

Constraints:

• The number of nodes in the tree is in the range [0, 5000].
• -1000 <= Node.val <= 1000
• -1000 <= targetSum <= 1000

A:

/**

* Definition for a binary tree node.

* struct TreeNode {

* int val;

* TreeNode *left;

* TreeNode *right;

* TreeNode() : val(0), left(nullptr), right(nullptr) {}

* TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}

* TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}

* };

*/

class Solution {

public:

bool hasPathSum(TreeNode* root, int targetSum) {

if(!root)

return false;

if( ! root->left && ! root->right)

return root->val == targetSum;

return hasPathSum(root->left, targetSum - root->val) || hasPathSum(root->right, targetSum - root->val);

}

};

----------3 rd pass ---------下面的这个代码是不对的，为什么呢？-----

public class Solution {
    public boolean hasPathSum(TreeNode root, int sum) {
        if(root==null)
            return false;
        return helper(root,sum);
    }
    private boolean helper(TreeNode root, int sum){
        if(root ==null)
            return sum==0;
        return helper(root.left, sum-root.val) || helper(root.right,sum-root.val);
    }
}

Mistakes:

1 :因为，对leaf的理解不对，只有当某个节点没有child的时候，它才是leaf。
而上面的方法，会让不是leaf的节点被当作节点
2:  即使是boolean类型的，要用的时候，也要  先显著的声明下，而不能用它的默认值。
（貌似避免不同平台的默认值不同）
Learned: