Brooke

一、題目

Given the root of a binary tree, return the sum of all left leaves.

A leaf is a node with no children. A left leaf is a leaf that is the left child of another node.

 

Example 1:

 

 

 

 

 

 

 

 

 

Input: root = [3,9,20,null,null,15,7] Output: 24 Explanation: There are two left leaves in the binary tree, with values 9 and 15 respectively.

Example 2:

Input: root = [1] Output: 0

 

Constraints:

• The number of nodes in the tree is in the range [1, 1000].
• -1000 <= Node.val <= 1000

 

二、程式作法

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     public int val;
 *     public TreeNode left;
 *     public TreeNode right;
 *     public TreeNode(int val=0, TreeNode left=null, TreeNode right=null) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
public class Solution
{
    public int sum = 0;
    public int SumOfLeftLeaves(TreeNode root)
    {
        Helper(root);
        return sum;
    }

    public void Helper(TreeNode root)
    {
        if (root.left != null && root.left.left == null && root.left.right == null) sum += root.left.val;
        if (root.left != null) Helper(root.left);
        if (root.right != null) Helper(root.right);
    }
}

 

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     public int val;
 *     public TreeNode left;
 *     public TreeNode right;
 *     public TreeNode(int val=0, TreeNode left=null, TreeNode right=null) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
public class Solution
{
    public int SumOfLeftLeaves(TreeNode root)
    {
        int sum = 0;
        if (root.left != null)
        {
            if (root.left.left == null && root.left.right == null)
                sum += root.left.val + SumOfLeftLeaves(root.left);
            else
                sum += SumOfLeftLeaves(root.left);
        }

        if (root.right != null)
            sum += SumOfLeftLeaves(root.right);
        return sum;
    }
}

 

三、思路筆記

注意，題目要的是左葉子的值，而不是左節點的值。