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150.evaluate-reverse-polish-notation.md

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+
+
+## 题目地址
+https://leetcode.com/problems/evaluate-reverse-polish-notation/description/
+
+## 题目描述
+
+```
+Evaluate the value of an arithmetic expression in Reverse Polish Notation.
+
+Valid operators are +, -, *, /. Each operand may be an integer or another expression.
+
+Note:
+
+Division between two integers should truncate toward zero.
+The given RPN expression is always valid. That means the expression would always evaluate to a result and there won't be any divide by zero operation.
+```
+## 思路
+逆波兰表达式又叫做后缀表达式。在通常的表达式中，二元运算符总是置于与之相关的两个运算对象之间，这种表示法也称为`中缀表示`。
+
+波兰逻辑学家J.Lukasiewicz于1929年提出了另一种表示表达式的方法，按此方法，每一运算符都置于其运算对象之后，故称为`后缀表示`。
+
+> 逆波兰表达式是一种十分有用的表达式，它将复杂表达式转换为可以依靠简单的操作得到计算结果的表达式。例如(a+b)*(c+d)转换为ab+cd+*
+
+
+## 关键点
+
+1. 栈的基本用法
+
+2. 如果你用的是JS的话，需要注意/ 和 其他很多语言是不一样的
+
+3. 如果你用的是JS的话，需要先将字符串转化为数字。否则有很多意想不到的结果
+
+4. 操作符的顺序应该是 先出栈的是第二位，后出栈的是第一位。 这在不符合交换律的操作中很重要， 比如减法和除法。
+
+## 代码
+
+```js
+/*
+ * @lc app=leetcode id=150 lang=javascript
+ *
+ * [150] Evaluate Reverse Polish Notation
+ *
+ * https://leetcode.com/problems/evaluate-reverse-polish-notation/description/
+ *
+ * algorithms
+ * Medium (31.43%)
+ * Total Accepted:    153.3K
+ * Total Submissions: 485.8K
+ * Testcase Example:  '["2","1","+","3","*"]'
+ *
+ * Evaluate the value of an arithmetic expression in Reverse Polish Notation.
+ *
+ * Valid operators are +, -, *, /. Each operand may be an integer or another
+ * expression.
+ *
+ * Note:
+ *
+ *
+ * Division between two integers should truncate toward zero.
+ * The given RPN expression is always valid. That means the expression would
+ * always evaluate to a result and there won't be any divide by zero
+ * operation.
+ *
+ *
+ * Example 1:
+ *
+ *
+ * Input: ["2", "1", "+", "3", "*"]
+ * Output: 9
+ * Explanation: ((2 + 1) * 3) = 9
+ *
+ *
+ * Example 2:
+ *
+ *
+ * Input: ["4", "13", "5", "/", "+"]
+ * Output: 6
+ * Explanation: (4 + (13 / 5)) = 6
+ *
+ *
+ * Example 3:
+ *
+ *
+ * Input: ["10", "6", "9", "3", "+", "-11", "*", "/", "*", "17", "+", "5", "+"]
+ * Output: 22
+ * Explanation:
+ * ⁠ ((10 * (6 / ((9 + 3) * -11))) + 17) + 5
+ * = ((10 * (6 / (12 * -11))) + 17) + 5
+ * = ((10 * (6 / -132)) + 17) + 5
+ * = ((10 * 0) + 17) + 5
+ * = (0 + 17) + 5
+ * = 17 + 5
+ * = 22
+ *
+ *
+ */
+/**
+ * @param {string[]} tokens
+ * @return {number}
+ */
+var evalRPN = function(tokens) {
+  const stack = [];
+
+  for (let index = 0; index < tokens.length; index++) {
+    const token = tokens[index];
+    if (!Number.isNaN(Number(token))) {
+      stack.push(token);
+    } else {
+      const a = Number(stack.pop());
+      const b = Number(stack.pop());
+      if (token === "*") {
+        stack.push(a * b);
+      } else if (token === "/") {
+        stack.push(b / a > 0 ? Math.floor(b / a) : Math.ceil(b / a));
+      } else if (token === "+") {
+        stack.push(a + b);
+      } else if (token === "-") {
+        stack.push(b - a);
+      }
+    }
+  }
+
+  return stack.pop();
+};
+
+```
+
+
+

203.remove-linked-list-elements.md

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+## 题目地址
+https://leetcode.com/problems/remove-linked-list-elements/description/
+
+## 题目描述
+```
+Remove all elements from a linked list of integers that have value val.
+
+Example:
+
+Input:  1->2->6->3->4->5->6, val = 6
+Output: 1->2->3->4->5
+
+```
+
+## 思路
+这个一个链表基本操作的题目，思路就不多说了。
+## 关键点解析
+
+- 链表的基本操作（删除指定节点）
+- 虚拟节点dummy 简化操作
+
+> 其实设置dummy节点就是为了处理特殊位置（头节点），这这道题就是如果头节点是给定的需要删除的节点呢？
+为了保证代码逻辑的一致性，即不需要为头节点特殊定制逻辑，才采用的虚拟节点。
+
+- 如果连续两个节点都是要删除的节点，这个情况容易被忽略。
+eg:
+
+```js
+// 只有下个节点不是要删除的节点才更新current
+if (!next || next.val !== val) {
+    current =  next;
+}
+
+```
+
+
+## 代码
+
+```js
+
+
+
+/*
+ * @lc app=leetcode id=203 lang=javascript
+ *
+ * [203] Remove Linked List Elements
+ *
+ * https://leetcode.com/problems/remove-linked-list-elements/description/
+ *
+ * algorithms
+ * Easy (35.32%)
+ * Total Accepted:    211.9K
+ * Total Submissions: 598.6K
+ * Testcase Example:  '[1,2,6,3,4,5,6]\n6'
+ *
+ * Remove all elements from a linked list of integers that have value val.
+ * 
+ * Example:
+ * 
+ * 
+ * Input:  1->2->6->3->4->5->6, val = 6
+ * Output: 1->2->3->4->5
+ * 
+ * 
+ */
+/**
+ * Definition for singly-linked list.
+ * function ListNode(val) {
+ *     this.val = val;
+ *     this.next = null;
+ * }
+ */
+/**
+ * @param {ListNode} head
+ * @param {number} val
+ * @return {ListNode}
+ */
+var removeElements = function(head, val) {
+    const dummy = {
+        next: head
+    }
+    let current = dummy;
+
+    while(current && current.next) {
+        let next = current.next;
+        if (next.val === val) {
+            current.next = next.next;
+            next = next.next;
+        }
+
+        if (!next || next.val !== val) {
+            current =  next;
+        }
+    }
+
+    return dummy.next;
+};
+
+```
+

219.contains-duplicate-ii.md

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+
+## 题目地址
+https://leetcode.com/problems/contains-duplicate-ii/description/
+
+## 题目描述
+
+
+```
+Given an array of integers and an integer k, find out whether there are two distinct indices i and j in the array such that nums[i] = nums[j] and the absolute difference between i and j is at most k.
+
+Example 1:
+
+Input: nums = [1,2,3,1], k = 3
+Output: true
+Example 2:
+
+Input: nums = [1,0,1,1], k = 1
+Output: true
+Example 3:
+
+Input: nums = [1,2,3,1,2,3], k = 2
+Output: false
+
+```
+
+## 思路
+
+由于题目没有对空间复杂度有求，用一个hashmap  存储已经访问过的数字即可,
+每次访问都会看hashmap中是否有这个元素，有的话拿出索引进行比对，是否满足条件（相隔不大于k），如果满足返回true即可。
+
+
+## 关键点解析
+
+无
+
+
+## 代码
+
+```js
+/*
+ * @lc app=leetcode id=219 lang=javascript
+ *
+ * [219] Contains Duplicate II
+ *
+ * https://leetcode.com/problems/contains-duplicate-ii/description/
+ *
+ * algorithms
+ * Easy (34.75%)
+ * Total Accepted:    187.3K
+ * Total Submissions: 537.5K
+ * Testcase Example:  '[1,2,3,1]\n3'
+ *
+ * Given an array of integers and an integer k, find out whether there are two
+ * distinct indices i and j in the array such that nums[i] = nums[j] and the
+ * absolute difference between i and j is at most k.
+ * 
+ * 
+ * Example 1:
+ * 
+ * 
+ * Input: nums = [1,2,3,1], k = 3
+ * Output: true
+ * 
+ * 
+ * 
+ * Example 2:
+ * 
+ * 
+ * Input: nums = [1,0,1,1], k = 1
+ * Output: true
+ * 
+ * 
+ * 
+ * Example 3:
+ * 
+ * 
+ * Input: nums = [1,2,3,1,2,3], k = 2
+ * Output: false
+ * 
+ * 
+ * 
+ * 
+ * 
+ */
+/**
+ * @param {number[]} nums
+ * @param {number} k
+ * @return {boolean}
+ */
+var containsNearbyDuplicate = function(nums, k) {
+    const visited = {};
+    for(let i = 0; i < nums.length; i++) {
+        const num = nums[i];
+        if (visited[num] !== undefined && i - visited[num] <= k) {
+            return true;
+        }
+        visited[num] = i;
+    }
+    return false
+};
+```
+

279.perfect-squares.js

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+/*
+ * @lc app=leetcode id=279 lang=javascript
+ *
+ * [279] Perfect Squares
+ *
+ * https://leetcode.com/problems/perfect-squares/description/
+ *
+ * algorithms
+ * Medium (40.98%)
+ * Total Accepted:    168.2K
+ * Total Submissions: 408.5K
+ * Testcase Example:  '12'
+ *
+ * Given a positive integer n, find the least number of perfect square numbers
+ * (for example, 1, 4, 9, 16, ...) which sum to n.
+ * 
+ * Example 1:
+ * 
+ * 
+ * Input: n = 12
+ * Output: 3 
+ * Explanation: 12 = 4 + 4 + 4.
+ * 
+ * Example 2:
+ * 
+ * 
+ * Input: n = 13
+ * Output: 2
+ * Explanation: 13 = 4 + 9.
+ */
+/**
+ * @param {number} n
+ * @return {number}
+ */
+var numSquares = function(n) {
+
+};
+