feat: azl397985856#15 azl397985856#454

Initiativethinker · May 3, 2019 · edf7916 · edf7916
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diff --git a/README.md b/README.md
@@ -69,23 +69,25 @@ leetcode 题解，记录自己的 leetcode 解题之路。
 
 ![koko-eating-bananas](./assets/problems/koko-eating-bananas.png)
 
-## 进度
+## Top题目进度
 
-- [Top 100 Linked Questions](https://leetcode.com/problemset/top-100-liked-questions/)(44/100)
+- [Top 100 Linked Questions](https://leetcode.com/problemset/top-100-liked-questions/) (44 / 100)
 
-- [Top Interview Questions](https://leetcode.com/problemset/top-interview-questions/)(59/145)
+- [Top Interview Questions](https://leetcode.com/problemset/top-interview-questions/) (64 / 145)
 ## 传送门
 
 ### leetcode 经典题目的解析
 
+> 这里仅列举具有代表性题目，并不是全部题目
+
 #### 简单难度
 
 - [0020.Valid Parentheses](./problems/20.validParentheses.md) 🖊 
 - [0026.remove-duplicates-from-sorted-array](./problems/26.remove-duplicates-from-sorted-array.md)
-- [0088.merge-sorted-array](./problems/88.merge-sorted-array.md) 🆕 
+- [0088.merge-sorted-array](./problems/88.merge-sorted-array.md)
 - [0136.single-number](./problems/136.single-number.md)
 - [0167.two-sum-ii-input-array-is-sorted](./problems/167.two-sum-ii-input-array-is-sorted.md)
-- [0169.majority-element](./problems/169.majority-element.md) 🆕 
+- [0169.majority-element](./problems/169.majority-element.md)
 - [0190.reverse-bits](./problems/190.reverse-bits.md)
 - [0191.number-of-1-bits](./problems/191.number-of-1-bits.md)
 - [0203.remove-linked-list-elements](./problems/203.remove-linked-list-elements.md)
@@ -101,12 +103,13 @@ leetcode 题解，记录自己的 leetcode 解题之路。
 - [0002. Add Two Numbers](./problems/2.addTwoNumbers.md)
 - [0003. Longest Substring Without Repeating Characters](./problems/3.longestSubstringWithoutRepeatingCharacters.md)
 - [0011.container-with-most-water](./problems/11.container-with-most-water.md)
+- [0015.3-sum](./problems/15.3-sum.md) 🆕
 - [0019. Remove Nth Node From End of List](./problems/19.removeNthNodeFromEndofList.md)
 - [0024. Swap Nodes In Pairs](./problems/24.swapNodesInPairs.md)
-- [0039.combination-sum](./problems/39.combination-sum.md) 🆕 
-- [0040.combination-sum-ii](./problems/40.combination-sum-ii.md) 🆕 
-- [0046.permutations](./problems/46.permutations.md) 🆕 
-- [0047.permutations-ii](./problems/47.permutations-ii.md) 🆕 
+- [0039.combination-sum](./problems/39.combination-sum.md)
+- [0040.combination-sum-ii](./problems/40.combination-sum-ii.md)
+- [0046.permutations](./problems/46.permutations.md)
+- [0047.permutations-ii](./problems/47.permutations-ii.md)
 - [0055.jump-game](./problems/55.jump-game.md) 🆕 
 - [0062.unique-paths](./problems/62.unique-paths.md )🆕 
 - [0075.sort-colors](./problems/75.sort-colors.md)
@@ -134,6 +137,7 @@ leetcode 题解，记录自己的 leetcode 解题之路。
 - [0328.odd-even-linked-list](./problems/328.odd-even-linked-list.md)
 - [0416.partition-equal-subset-sum](./problems/416.partition-equal-subset-sum.md)
 - [0445.add-two-numbers-ii](./problems/445.add-two-numbers-ii.md)
+- [0454.4-sum-ii](./problems/454.4-sum-ii.md) 🆕
 - [0518.coin-change-2](./problems/518.coin-change-2.md)
 - [0875.koko-eating-bananas](./problems/875.koko-eating-bananas.md)
 - [0877.stone-game](./problems/877.stone-game.md)
@@ -205,3 +209,9 @@ anki - 文件 - 导入 - 下拉格式选择“打包的 anki集合”，然后
 ![wechat-group-chat](./assets/wechat-group-chat.jpeg)
 
 (由于微信的限制，100个人以上只能邀请加入， 你可以添加我的机器人回复“leetcode”拉你进群)
+
+## 贡献
+
+- 如果有想法和创意，请提[issue](https://github.com/azl397985856/leetcode/issues)或者进群提
+- 如果想贡献代码，请提[PR](https://github.com/azl397985856/leetcode/pulls)
+- 如果需要修改项目中图片，[这里](./assets/drawio/)存放了项目中绘制图的源代码， 大家可以用[draw.io](https://www.draw.io/)打开进行编辑。
diff --git a/assets/drawio/15.3-sum.drawio b/assets/drawio/15.3-sum.drawio
@@ -0,0 +1 @@
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diff --git a/assets/drawio/454.4-sum-ii.drawio b/assets/drawio/454.4-sum-ii.drawio
@@ -0,0 +1 @@
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diff --git a/assets/problems/15.3-sum.png b/assets/problems/15.3-sum.png
diff --git a/assets/problems/454.4-sum-ii.png b/assets/problems/454.4-sum-ii.png
diff --git a/problems/15.3-sum.md b/problems/15.3-sum.md
@@ -0,0 +1,127 @@
+## 题目地址
+https://leetcode.com/problems/3sum/description/
+
+## 题目描述
+```
+Given an array nums of n integers, are there elements a, b, c in nums such that a + b + c = 0? Find all unique triplets in the array which gives the sum of zero.
+
+Note:
+
+The solution set must not contain duplicate triplets.
+
+Example:
+
+Given array nums = [-1, 0, 1, 2, -1, -4],
+
+A solution set is:
+[
+ [-1, 0, 1],
+ [-1, -1, 2]
+]
+
+```
+## 思路
+
+我们采用`分治`的思想. 想要找出三个数相加等于0，我们可以数组依次遍历，
+每一项a[i]我们都认为它是最终能够用组成0中的一个数字，那么我们的目标就是找到
+剩下的元素（除a[i]）`两个`相加等于-a[i].
+
+通过上面的思路，我们的问题转化为了`给定一个数组，找出其中两个相加等于给定值`，
+这个问题是比较简单的， 我们只需要对数组进行排序，然后双指针解决即可。 加上我们需要外层遍历依次数组，因此总的时间复杂度应该是O(N^2)。
+
+思路如图所示：
+
+![15.3-sum](../assets/problems/15.3-sum.png)
+
+> 在这里之所以要排序解决是因为， 我们算法的瓶颈在这里不在于排序，而在于O(N^2)，如果我们瓶颈是排序，就可以考虑别的方式了
+
+
+> 如果找某一个特定元素，一个指针就够了。如果是找两个元素满足一定关系（比如求和等于特定值），需要双指针，
+当然前提是数组有序。
+## 关键点解析
+
+- 排序之后，用双指针
+- 分治
+
+## 代码
+```js
+
+/*
+ * @lc app=leetcode id=15 lang=javascript
+ *
+ * [15] 3Sum
+ *
+ * https://leetcode.com/problems/3sum/description/
+ *
+ * algorithms
+ * Medium (23.51%)
+ * Total Accepted: 531.5K
+ * Total Submissions: 2.2M
+ * Testcase Example: '[-1,0,1,2,-1,-4]'
+ *
+ * Given an array nums of n integers, are there elements a, b, c in nums such
+ * that a + b + c = 0? Find all unique triplets in the array which gives the
+ * sum of zero.
+ *
+ * Note:
+ *
+ * The solution set must not contain duplicate triplets.
+ *
+ * Example:
+ *
+ *
+ * Given array nums = [-1, 0, 1, 2, -1, -4],
+ *
+ * A solution set is:
+ * [
+ * ⁠ [-1, 0, 1],
+ * ⁠ [-1, -1, 2]
+ * ]
+ *
+ *
+ */
+/**
+ * @param {number[]} nums
+ * @return {number[][]}
+ */
+var threeSum = function(nums) {
+ if (nums.length < 3) return [];
+ const list = [];
+ nums.sort((a, b) => a - b);
+ for (let i = 0; i < nums.length; i++) {
+ // skip duplicated result without set
+ if (i > 0 && nums[i] === nums[i - 1]) continue;
+ let left = i;
+ let right = nums.length - 1;
+
+ // for each index i
+ // we want to find the triplet [i, left, right] which sum to 0
+ while (left < right) {
+ // skip i === left or i === right, in that case, the index i will be used twice
+ if (left === i) {
+ left++;
+ } else if (right === i) {
+ right--;
+ } else if (nums[left] + nums[right] + nums[i] === 0) {
+ list.push([nums[left], nums[right], nums[i]]);
+ // skip duplicated result without set
+ while(nums[left] === nums[left + 1]) {
+ left++;
+ }
+ left++;
+ // skip duplicated result without set
+ while(nums[right] === nums[right - 1]) {
+ right--;
+ }
+ right--;
+ continue;
+ } else if (nums[left] + nums[right] + nums[i] > 0) {
+ right--;
+ } else {
+ left++;
+ }
+ }
+ }
+ return list;
+};
+```
diff --git a/problems/2.addTwoNumbers.md b/problems/2.addTwoNumbers.md
@@ -2,12 +2,13 @@
 https://leetcode.com/problems/add-two-numbers/description/
 
 ## 题目描述
+```
 You are given two non-empty linked lists representing two non-negative integers. The digits are stored in reverse order and each of their nodes contain a single digit. Add the two numbers and return it as a linked list.
 
 You may assume the two numbers do not contain any leading zero, except the number 0 itself.
 
 Example
-```
+
 Input: (2 -> 4 -> 3) + (5 -> 6 -> 4)
 Output: 7 -> 0 -> 8
 Explanation: 342 + 465 = 807.

diff --git a/problems/236.lowest-common-ancestor-of-a-binary-tree.md b/problems/236.lowest-common-ancestor-of-a-binary-tree.md
@@ -148,5 +148,5 @@ var lowestCommonAncestor = function(root, p, q) {
 ```
 
 ## 扩展
-如果递归的结束条件改为`if (!root || root.left === p || root.right === q) return root;` 代表的是嗯呢么意思，对结果有什么样的影响？
+如果递归的结束条件改为`if (!root || root.left === p || root.right === q) return root;` 代表的是什么意思，对结果有什么样的影响？
 
diff --git a/problems/454.4-sum-ii.md b/problems/454.4-sum-ii.md
@@ -0,0 +1,116 @@
+
+
+## 题目地址
+https://leetcode.com/problems/4sum-ii/description/
+
+## 题目描述
+
+```
+Given four lists A, B, C, D of integer values, compute how many tuples (i, j, k, l) there are such that A[i] + B[j] + C[k] + D[l] is zero.
+
+To make problem a bit easier, all A, B, C, D have same length of N where 0 ≤ N ≤ 500. All integers are in the range of -228 to 228 - 1 and the result is guaranteed to be at most 231 - 1.
+
+Example:
+
+Input:
+A = [ 1, 2]
+B = [-2,-1]
+C = [-1, 2]
+D = [ 0, 2]
+
+Output:
+2
+
+Explanation:
+The two tuples are:
+1. (0, 0, 0, 1) -> A[0] + B[0] + C[0] + D[1] = 1 + (-2) + (-1) + 2 = 0
+2. (1, 1, 0, 0) -> A[1] + B[1] + C[0] + D[0] = 2 + (-1) + (-1) + 0 = 0
+```
+## 思路
+
+如果按照常规思路去完成查找需要四层遍历，时间复杂是O(n^4), 显然是行不通的。
+因此我们有必要想一种更加高效的算法。
+
+我一个思路就是我们将四个数组分成两组，两两结合。
+然后我们分别计算`两两结合能够算出的和有哪些，以及其对应的个数`。
+
+如图：
+
+![454.4-sum-ii](../assets/problems/454.4-sum-ii.png)
+
+
+这个时候我们得到了两个`hashTable`， 我们只需要进行简单的数学运算就可以得到结果。
+
+## 关键点解析
+
+
+
+## 代码
+```js
+
+/*
+ * @lc app=leetcode id=454 lang=javascript
+ *
+ * [454] 4Sum II
+ *
+ * https://leetcode.com/problems/4sum-ii/description/
+ *
+ * algorithms
+ * Medium (49.93%)
+ * Total Accepted: 63.2K
+ * Total Submissions: 125.6K
+ * Testcase Example: '[1,2]\n[-2,-1]\n[-1,2]\n[0,2]'
+ *
+ * Given four lists A, B, C, D of integer values, compute how many tuples (i,
+ * j, k, l) there are such that A[i] + B[j] + C[k] + D[l] is zero.
+ *
+ * To make problem a bit easier, all A, B, C, D have same length of N where 0 ≤
+ * N ≤ 500. All integers are in the range of -2^28 to 2^28 - 1 and the result
+ * is guaranteed to be at most 2^31 - 1.
+ *
+ * Example:
+ *
+ *
+ * Input:
+ * A = [ 1, 2]
+ * B = [-2,-1]
+ * C = [-1, 2]
+ * D = [ 0, 2]
+ *
+ * Output:
+ * 2
+ *
+ * Explanation:
+ * The two tuples are:
+ * 1. (0, 0, 0, 1) -> A[0] + B[0] + C[0] + D[1] = 1 + (-2) + (-1) + 2 = 0
+ * 2. (1, 1, 0, 0) -> A[1] + B[1] + C[0] + D[0] = 2 + (-1) + (-1) + 0 = 0
+ *
+ *
+ *
+ *
+ */
+/**
+ * @param {number[]} A
+ * @param {number[]} B
+ * @param {number[]} C
+ * @param {number[]} D
+ * @return {number}
+ */
+var fourSumCount = function(A, B, C, D) {
+ const sumMapper = {};
+ let res = 0;
+ for (let i = 0; i < A.length; i++) {
+ for (let j = 0; j < B.length; j++) {
+ sumMapper[A[i] + B[j]] = (sumMapper[A[i] + B[j]] || 0) + 1;
+ }
+ }
+
+ for (let i = 0; i < C.length; i++) {
+ for (let j = 0; j < D.length; j++) {
+ res += sumMapper[- (C[i] + D[j])] || 0;
+ }
+ }
+
+ return res;
+};
+```
diff --git a/thinkings/basic-data-structure.md b/thinkings/basic-data-structure.md
@@ -285,7 +285,7 @@ return, children, sibling也都是一个fiber，因此fiber看起来就是一个
 
 相关概念：
 
-- 真二叉树
+- 真二叉树 （所有节点的度数只能是偶数，即只能为0或者2）
 
 
 另外我也专门开设了[二叉树的遍历](./binary-tree-traversal.md)章节, 具体细节和算法可以去那里查看。

diff --git a/thinkings/dynamic-programming.md b/thinkings/dynamic-programming.md
@@ -155,10 +155,10 @@ f(n) = f(n-1) + f(n-2) 就是【状态转移公式】
 
 ### 相关问题
 
-[硬币找零问题](../problems/322.coin-change.md)
-[硬币找零问题2](../problems/518.coin-change-2.md)
-[分词问题](../problems/139.word-break.md)
-[416.partition-equal-subset-sum](../problems/416.partition-equal-subset-sum.md)
+- [硬币找零问题](../problems/322.coin-change.md)
+- [硬币找零问题2](../problems/518.coin-change-2.md)
+- [分词问题](../problems/139.word-break.md)
+- [416.partition-equal-subset-sum](../problems/416.partition-equal-subset-sum.md)
 
 ## 总结