Added solution for Challenge-1(Two Sum)

BuyStockSell/Solution.java

@@ -0,0 +1,12 @@
+class Solution {
+    public int maxProfit(int[] prices) {
+        int maxi=0;
+        int mini=prices[0];
+        for(int i=1;i<prices.length;i++){
+            int cost=prices[i]-mini;
+            maxi=Math.max(maxi,cost);
+            mini=Math.min(prices[i],mini);
+        }
+        return maxi;
+    }
+}

ContainsDuplicate/Solution.java

@@ -0,0 +1,13 @@
+class Solution {
+    public boolean containsDuplicate(int[] nums) {
+        Set<Integer> res=new HashSet<>();
+        for(int i:nums){
+            if(res.contains(i)){
+                return true;
+            }
+            res.add(i);
+        }
+        return false;
+
+    }
+}

TwoSum/two_sum.java

@@ -0,0 +1,18 @@
+class Solution {
+    public int[] twoSum(int[] nums, int target) {
+        HashMap<Integer,Integer> hashMap=new HashMap<Integer,Integer>();
+        int []index=new int[2];
+        for(int i=0;i<nums.length;i++){
+            int curNum=nums[i];
+            int match=target-curNum;
+            if(hashMap.containsKey(match)){
+                index[0]=i;
+                index[1]=hashMap.get(match);
+            }else{
+                hashMap.put(curNum,i);
+            }
+        }
+        return index;
+
+    }
+}