add Easy_404_Sum_of_Left_Leaves

Author: zongyanqi

Easy_404_Sum_of_Left_Leaves.js

@@ -0,0 +1,41 @@
+/**
+ * Find the sum of all left leaves in a given binary tree.
+
+ Example:
+
+ 3
+ / \
+ 9  20
+ /  \
+ 15   7
+
+ There are two left leaves in the binary tree, with values 9 and 15 respectively. Return 24.
+ *
+ */
+
+/**
+ * Definition for a binary tree node.
+ * function TreeNode(val) {
+ *     this.val = val;
+ *     this.left = this.right = null;
+ * }
+ */
+
+/**
+ * @param {TreeNode} root
+ * @return {number}
+ */
+var sumOfLeftLeaves = function (root) {
+    return fn(root, false);
+};
+
+function fn(node, isLeft) {
+    if (!node) return 0;
+    if (!node.left && !node.right) {
+        return isLeft ? node.val : 0;
+    }
+
+    return fn(node.left, true) + fn(node.right, false);
+
+}
+