添加（0404.左叶子之和.md）：增加typescript版本

problems/0404.左叶子之和.md

@@ -372,6 +372,50 @@ var sumOfLeftLeaves = function(root) {
 };
 ```
 
+## TypeScript
+
+> 递归法
+
+```typescript
+function sumOfLeftLeaves(root: TreeNode | null): number {
+    if (root === null) return 0;
+    let midVal: number = 0;
+    if (
+        root.left !== null &&
+        root.left.left === null &&
+        root.left.right === null
+    ) {
+        midVal = root.left.val;
+    }
+    let leftVal: number = sumOfLeftLeaves(root.left);
+    let rightVal: number = sumOfLeftLeaves(root.right);
+    return midVal + leftVal + rightVal;
+};
+```
+
+> 迭代法
+
+```typescript
+function sumOfLeftLeaves(root: TreeNode | null): number {
+    let helperStack: TreeNode[] = [];
+    let tempNode: TreeNode;
+    let sum: number = 0;
+    if (root !== null) helperStack.push(root);
+    while (helperStack.length > 0) {
+        tempNode = helperStack.pop()!;
+        if (
+            tempNode.left !== null &&
+            tempNode.left.left === null &&
+            tempNode.left.right === null
+        ) {
+            sum += tempNode.left.val;
+        }
+        if (tempNode.right !== null) helperStack.push(tempNode.right);
+        if (tempNode.left !== null) helperStack.push(tempNode.left);
+    }
+    return sum;
+};
+```
 
 ## Swift