添加0404.左叶子之和.md层序迭代解法Java代码

problems/0404.左叶子之和.md

@@ -201,7 +201,31 @@ class Solution {
     }
 }
 ```
-
+```java
+// 层序遍历迭代法
+class Solution {
+    public int sumOfLeftLeaves(TreeNode root) {
+        int sum = 0;
+        if (root == null) return 0;
+        Queue<TreeNode> queue = new LinkedList<>();
+        queue.offer(root);
+        while (!queue.isEmpty()) {
+            int size = queue.size();
+            while (size -- > 0) {
+                TreeNode node = queue.poll();
+                if (node.left != null) { // 左节点不为空
+                    queue.offer(node.left);
+                    if (node.left.left == null && node.left.right == null){ // 左叶子节点
+                        sum += node.left.val;
+                    }
+                }
+                if (node.right != null) queue.offer(node.right);
+            }
+        }
+        return sum;
+    }
+}
+```
 
 
 ## Python