diff --git a/.gitignore b/.gitignore
new file mode 100644
index 000000000..496ee2ca6
--- /dev/null
+++ b/.gitignore
@@ -0,0 +1 @@
+.DS_Store
\ No newline at end of file
diff --git a/README.md b/README.md
index f61d03de6..9a0a55a7c 100644
--- a/README.md
+++ b/README.md
@@ -6,15 +6,15 @@ leetcode 题解，记录自己的 leecode 解题之路。
 
 ### 简单难度
 
-- [20. Valid Parentheses](https://github.com/azl397985856/leetcode/blob/master/validParentheses.md)
-- [26.remove-duplicates-from-sorted-array](https://github.com/azl397985856/leetcode/blob/master/26.remove-duplicates-from-sorted-array.md)
-- [206.reverse-linked-list](./206.reverse-linked-list.md)
-- [136.single-number](./136.single-number.md)
-- [167.two-sum-ii-input-array-is-sorted](./167.two-sum-ii-input-array-is-sorted.md)
-- [203.remove-linked-list-elements](./203.remove-linked-list-elements.md)
-- [219.contains-duplicate-ii](./219.contains-duplicate-ii.md)
-- [283.move-zeroes](./283.move-zeroes.md)
-- [349.intersection-of-two-arrays](./349.intersection-of-two-arrays.md)
+- [20. Valid Parentheses](./problems/validParentheses.md)
+- [26.remove-duplicates-from-sorted-array](./problems/26.remove-duplicates-from-sorted-array.md)
+- [206.reverse-linked-list](./problems/206.reverse-linked-list.md)
+- [136.single-number](./problems/136.single-number.md)
+- [167.two-sum-ii-input-array-is-sorted](./problems/167.two-sum-ii-input-array-is-sorted.md)
+- [203.remove-linked-list-elements](./problems/203.remove-linked-list-elements.md)
+- [219.contains-duplicate-ii](./problems/219.contains-duplicate-ii.md)
+- [283.move-zeroes](./problems/283.move-zeroes.md)
+- [349.intersection-of-two-arrays](./problems/349.intersection-of-two-arrays.md)
 
 ### 中等难度
 
@@ -24,21 +24,21 @@ leetcode 题解，记录自己的 leecode 解题之路。
 - [19. Remove Nth Node From End of List](https://github.com/azl397985856/leetcode/blob/master/removeNthNodeFromEndofList.md)
 - [24. Swap Nodes In Pairs](https://github.com/azl397985856/leetcode/blob/master/swapNodesInPairs.md)
 - [75.sort-colors.md](https://github.com/azl397985856/leetcode/blob/master/75.sort-colors.md)
-- [86.partition-list](./86.partition-list.md)
-- [92.reverse-linked-list-ii](./92.reverse-linked-list-ii.md)
-- [94.binary-tree-inorder-traversal](./94.binary-tree-inorder-traversal.md)
-- [102.binary-tree-level-order-traversal](./102.binary-tree-level-order-traversal.md)
-- [103.binary-tree-zigzag-level-order-traversal](./103.binary-tree-zigzag-level-order-traversal.md)
-- [144.binary-tree-preorder-traversal](./144.binary-tree-preorder-traversal.md)
-- [150.evaluate-reverse-polish-notation](./150.evaluate-reverse-polish-notation.md)
-- [328.odd-even-linked-list](./328.odd-even-linked-list.md)
-- [445.add-two-numbers-ii](./445.add-two-numbers-ii.md)
-- [877.stone-game](./877.stone-game.md)
-- [279.perfect-squares](./279.perfect-squares.md)
-- [199.binary-tree-right-side-view](./199.binary-tree-right-side-view.md)
-- [209.minimum-size-subarray-sum](./209.minimum-size-subarray-sum.md)
+- [86.partition-list](./problems/86.partition-list.md)
+- [92.reverse-linked-list-ii](./problems/92.reverse-linked-list-ii.md)
+- [94.binary-tree-inorder-traversal](./problems/94.binary-tree-inorder-traversal.md)
+- [102.binary-tree-level-order-traversal](./problems/102.binary-tree-level-order-traversal.md)
+- [103.binary-tree-zigzag-level-order-traversal](./problems/103.binary-tree-zigzag-level-order-traversal.md)
+- [144.binary-tree-preorder-traversal](./problems/144.binary-tree-preorder-traversal.md)
+- [150.evaluate-reverse-polish-notation](./problems/150.evaluate-reverse-polish-notation.md)
+- [328.odd-even-linked-list](./problems/328.odd-even-linked-list.md)
+- [445.add-two-numbers-ii](./problems/445.add-two-numbers-ii.md)
+- [877.stone-game](./problems/877.stone-game.md)
+- [279.perfect-squares](./problems/279.perfect-squares.md)
+- [199.binary-tree-right-side-view](./problems/199.binary-tree-right-side-view.md)
+- [209.minimum-size-subarray-sum](./problems/209.minimum-size-subarray-sum.md)
 
 ### 困难难度
 
-- [145.binary-tree-postorder-traversal](./145.binary-tree-postorder-traversal.md)
-- [146.lru-cache](./146.lru-cache.md)
+- [145.binary-tree-postorder-traversal](./problems/145.binary-tree-postorder-traversal.md)
+- [146.lru-cache](./problems/146.lru-cache.md)
diff --git a/102.binary-tree-level-order-traversal.md b/problems/102.binary-tree-level-order-traversal.md
similarity index 100%
rename from 102.binary-tree-level-order-traversal.md
rename to problems/102.binary-tree-level-order-traversal.md
diff --git a/103.binary-tree-zigzag-level-order-traversal.md b/problems/103.binary-tree-zigzag-level-order-traversal.md
similarity index 100%
rename from 103.binary-tree-zigzag-level-order-traversal.md
rename to problems/103.binary-tree-zigzag-level-order-traversal.md
diff --git a/136.single-number.md b/problems/136.single-number.md
similarity index 100%
rename from 136.single-number.md
rename to problems/136.single-number.md
diff --git a/144.binary-tree-preorder-traversal.md b/problems/144.binary-tree-preorder-traversal.md
similarity index 100%
rename from 144.binary-tree-preorder-traversal.md
rename to problems/144.binary-tree-preorder-traversal.md
diff --git a/145.binary-tree-postorder-traversal.md b/problems/145.binary-tree-postorder-traversal.md
similarity index 100%
rename from 145.binary-tree-postorder-traversal.md
rename to problems/145.binary-tree-postorder-traversal.md
diff --git a/146.lru-cache.md b/problems/146.lru-cache.md
similarity index 100%
rename from 146.lru-cache.md
rename to problems/146.lru-cache.md
diff --git a/150.evaluate-reverse-polish-notation.md b/problems/150.evaluate-reverse-polish-notation.md
similarity index 100%
rename from 150.evaluate-reverse-polish-notation.md
rename to problems/150.evaluate-reverse-polish-notation.md
diff --git a/167.two-sum-ii-input-array-is-sorted.md b/problems/167.two-sum-ii-input-array-is-sorted.md
similarity index 100%
rename from 167.two-sum-ii-input-array-is-sorted.md
rename to problems/167.two-sum-ii-input-array-is-sorted.md
diff --git a/199.binary-tree-right-side-view.md b/problems/199.binary-tree-right-side-view.md
similarity index 100%
rename from 199.binary-tree-right-side-view.md
rename to problems/199.binary-tree-right-side-view.md
diff --git a/203.remove-linked-list-elements.md b/problems/203.remove-linked-list-elements.md
similarity index 100%
rename from 203.remove-linked-list-elements.md
rename to problems/203.remove-linked-list-elements.md
diff --git a/206.reverse-linked-list.md b/problems/206.reverse-linked-list.md
similarity index 100%
rename from 206.reverse-linked-list.md
rename to problems/206.reverse-linked-list.md
diff --git a/209.minimum-size-subarray-sum.md b/problems/209.minimum-size-subarray-sum.md
similarity index 100%
rename from 209.minimum-size-subarray-sum.md
rename to problems/209.minimum-size-subarray-sum.md
diff --git a/219.contains-duplicate-ii.md b/problems/219.contains-duplicate-ii.md
similarity index 100%
rename from 219.contains-duplicate-ii.md
rename to problems/219.contains-duplicate-ii.md
diff --git a/26.remove-duplicates-from-sorted-array.md b/problems/26.remove-duplicates-from-sorted-array.md
similarity index 100%
rename from 26.remove-duplicates-from-sorted-array.md
rename to problems/26.remove-duplicates-from-sorted-array.md
diff --git a/279.perfect-squares.md b/problems/279.perfect-squares.md
similarity index 100%
rename from 279.perfect-squares.md
rename to problems/279.perfect-squares.md
diff --git a/283.move-zeroes.md b/problems/283.move-zeroes.md
similarity index 100%
rename from 283.move-zeroes.md
rename to problems/283.move-zeroes.md
diff --git a/328.odd-even-linked-list.md b/problems/328.odd-even-linked-list.md
similarity index 100%
rename from 328.odd-even-linked-list.md
rename to problems/328.odd-even-linked-list.md
diff --git a/349.intersection-of-two-arrays.md b/problems/349.intersection-of-two-arrays.md
similarity index 100%
rename from 349.intersection-of-two-arrays.md
rename to problems/349.intersection-of-two-arrays.md
diff --git a/445.add-two-numbers-ii.md b/problems/445.add-two-numbers-ii.md
similarity index 100%
rename from 445.add-two-numbers-ii.md
rename to problems/445.add-two-numbers-ii.md
diff --git a/75.sort-colors.md b/problems/75.sort-colors.md
similarity index 100%
rename from 75.sort-colors.md
rename to problems/75.sort-colors.md
diff --git a/86.partition-list.md b/problems/86.partition-list.md
similarity index 100%
rename from 86.partition-list.md
rename to problems/86.partition-list.md
diff --git a/877.stone-game.md b/problems/877.stone-game.md
similarity index 100%
rename from 877.stone-game.md
rename to problems/877.stone-game.md
diff --git a/92.reverse-linked-list-ii.md b/problems/92.reverse-linked-list-ii.md
similarity index 100%
rename from 92.reverse-linked-list-ii.md
rename to problems/92.reverse-linked-list-ii.md
diff --git a/94.binary-tree-inorder-traversal.md b/problems/94.binary-tree-inorder-traversal.md
similarity index 100%
rename from 94.binary-tree-inorder-traversal.md
rename to problems/94.binary-tree-inorder-traversal.md
diff --git a/addTwoNumbers.md b/problems/addTwoNumbers.md
similarity index 100%
rename from addTwoNumbers.md
rename to problems/addTwoNumbers.md
diff --git a/longestPalindromicSubstring.md b/problems/longestPalindromicSubstring.md
similarity index 100%
rename from longestPalindromicSubstring.md
rename to problems/longestPalindromicSubstring.md
diff --git a/longestSubstringWithoutRepeatingCharacters.md b/problems/longestSubstringWithoutRepeatingCharacters.md
similarity index 100%
rename from longestSubstringWithoutRepeatingCharacters.md
rename to problems/longestSubstringWithoutRepeatingCharacters.md
diff --git a/removeNthNodeFromEndofList.md b/problems/removeNthNodeFromEndofList.md
similarity index 100%
rename from removeNthNodeFromEndofList.md
rename to problems/removeNthNodeFromEndofList.md
diff --git a/swapNodesInPairs.md b/problems/swapNodesInPairs.md
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rename to problems/swapNodesInPairs.md
diff --git a/validParentheses.md b/problems/validParentheses.md
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