Two Sum

Author: applewjg

TwoSum.java

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+/*
+ Author:     King, [email protected]
+ Date:       Dec 13, 2014
+ Problem:    Two Sum
+ Difficulty: Medium
+ Source:     https://oj.leetcode.com/problems/two-sum/
+ Notes:
+ Given an array of integers, 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. Please note that your returned answers (both index1 and index2) are not zero-based.
+
+ You may assume that each input would have exactly one solution.
+
+ Input: numbers={2, 7, 11, 15}, target=9
+ Output: index1=1, index2=2
+
+ Solution: 1. Hash table. O(n)
+           
+ Note:  Hash Table solution has been updated.  In case that the two elements are the same, 
+        all the indices should be stored in the map.
+ */
+
+public class Solution {
+    public int[] twoSum(int[] numbers, int target) {
+        HashMap<Integer, Integer> map = new HashMap<Integer, Integer>();
+        for (int i = 0; i < numbers.length; ++i) {
+            int b = target - numbers[i];
+            if (map.get(b) != null) {
+                return new int[]{map.get(b),i+1};
+            } else map.put(numbers[i],i+1);
+        }
+        return null;
+    }
+}