To find the left leaves. Use recursive function to traversal all of the nodes, and find and sum the left leaf

```
public int sumOfLeftLeaves(TreeNode root) {
if (root == null) {
return 0;
}
int sum = 0;
// Check the left node is a leaf or not. If yes, add it to sum and return the sum
if (root.left != null && root.left.left == null && root.left.right == null) {
sum += root.left.val;
}
sum += sumOfLeftLeaves(root.left);
sum += sumOfLeftLeaves(root.right);
return sum;
}
```