two sum ii

167.two-sum-ii-input-array-is-sorted.md

+
+## 题目地址
+https://leetcode.com/problems/two-sum-ii-input-array-is-sorted/description/
+
+## 题目描述
+
+这是leetcode头号题目`two sum`的第二个版本，难度简单。
+
+```
+Given an array of integers that is already sorted in ascending order, find two numbers such that they add up to a specific target number.
+
+The function twoSum should return indices of the two numbers such that they add up to the target, where index1 must be less than index2.
+
+Note:
+
+Your returned answers (both index1 and index2) are not zero-based.
+You may assume that each input would have exactly one solution and you may not use the same element twice.
+Example:
+
+Input: numbers = [2,7,11,15], target = 9
+Output: [1,2]
+Explanation: The sum of 2 and 7 is 9. Therefore index1 = 1, index2 = 2.
+
+```
+
+## 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。
+
+假如题目空间复杂度有要求，由于数组是有序的，只需要双指针即可。一个left指针，一个right指针，
+如果left + right 值 大于target 则 right左移动， 否则left右移，代码比较简单， 不贴了。
+
+> 如果数组无序，需要先排序（从这里也可以看出排序是多么重要的操作）
+
+
+## 关键点解析
+
+无
+
+
+## 代码
+
+```js
+/*
+ * @lc app=leetcode id=167 lang=javascript
+ *
+ * [167] Two Sum II - Input array is sorted
+ *
+ * https://leetcode.com/problems/two-sum-ii-input-array-is-sorted/description/
+ *
+ * algorithms
+ * Easy (49.46%)
+ * Total Accepted:    221.8K
+ * Total Submissions: 447K
+ * Testcase Example:  '[2,7,11,15]\n9'
+ *
+ * Given an array of integers that is already sorted in ascending order, find
+ * two numbers such that they add up to a specific target number.
+ * 
+ * The function twoSum should return indices of the two numbers such that they
+ * add up to the target, where index1 must be less than index2.
+ * 
+ * Note:
+ * 
+ * 
+ * Your returned answers (both index1 and index2) are not zero-based.
+ * You may assume that each input would have exactly one solution and you may
+ * not use the same element twice.
+ * 
+ * 
+ * Example:
+ * 
+ * 
+ * Input: numbers = [2,7,11,15], target = 9
+ * Output: [1,2]
+ * Explanation: The sum of 2 and 7 is 9. Therefore index1 = 1, index2 = 2.
+ * 
+ */
+/**
+ * @param {number[]} numbers
+ * @param {number} target
+ * @return {number[]}
+ */
+var twoSum = function(numbers, target) {
+    const visited = {} // 记录出现的数字， 空间复杂度N
+
+    for (let index = 0; index < numbers.length; index++) {
+        const element = numbers[index];
+        if (visited[target - element]) {
+            return [visited[target - element], index + 1]
+        }
+        visited[element] = index + 1;   
+    }
+    return [];
+};
+
+
+```
+
+
+