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# Sum of Left Leaves

<https://leetcode.com/problems/sum-of-left-leaves/description/>

> Find the sum of all left leaves in a given binary tree

## Thoughts

每个节点判断左儿子是否是叶子, 是就把它值加上.

## Code

```
/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public int sumOfLeftLeaves(TreeNode root) {
        int sum = 0;
        if (root == null) {
            return sum;
        }
        Queue<TreeNode> queue = new LinkedList<>();
        queue.add(root);
        while (!queue.isEmpty()) {          
            int length = queue.size();
            for (int i = 0; i < length; i++) {
                TreeNode node = queue.poll();
                if (node.left != null) {
                    queue.add(node.left);     
                }
                if (node.right != null) {
                    queue.add(node.right);
                }
                if (node.left != null && node.left.left == null && node.left.right == null) {
                    sum += node.left.val;
                }
            }
        }

        return sum;
    }
}
```

## Analysis

时间复杂度O(n).