# Problem 404: Sum of Left Leaves

> [https://leetcode.com/problems/sum-of-left-leaves/#/description](https://leetcode.com/problems/sum-of-left-leaves/#/description)

## 思路

* 主要的思路就是学会遍历整个 tree，这样就可以进行各种操作
* recursion&#x20;

```java
/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public int sumOfLeftLeaves(TreeNode root) {
        if (root == null) return 0;
        int sum = 0;

        if (root.left != null) {
            if (root.left.left == null && root.left.right == null) {
                sum += root.left.val;
            } else {
                sum += sumOfLeftLeaves(root.left);
            }
        }
        sum += sumOfLeftLeaves(root.right);

        return sum;
    }
}
```

* iterative

```java
/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public int sumOfLeftLeaves(TreeNode root) {
        if (root == null) return 0;
        int sum = 0;

        Stack<TreeNode> stack = new Stack<>();
        stack.push(root);
        while (!stack.isEmpty()) {
            TreeNode node = stack.pop();
            if (node.left != null) {
                if (node.left.left == null && node.left.right == null) {
                    sum += node.left.val;
                } else {
                    stack.push(node.left);
                }
            }
            if (node.right != null) {
                if (node.right.left != null || node.right.right != null) stack.push(node.right);
            }
        }

        return sum;
    }
}
```
