edit 523

Author: fishercoder1534

README.md

@@ -110,7 +110,7 @@ Your ideas/fixes/algorithms are more than welcome!
 |526|[Beautiful Arrangement](https://leetcode.com/problems/beautiful-arrangement/)|[Solution](../master/src/main/java/com/fishercoder/solutions/_526.java) | O(n) |O(h) | Medium | Backtracking
 |525|[Contiguous Array](https://leetcode.com/problems/contiguous-array/)|[Solution](../master/src/main/java/com/fishercoder/solutions/_525.java) | O(n) |O(n) | Medium | HashMap
 |524|[Longest Word in Dictionary through Deleting](https://leetcode.com/problems/longest-word-in-dictionary-through-deleting/)|[Solution](../master/src/main/java/com/fishercoder/solutions/_524.java) | O(n) |O(n) | Medium | Sort
-|523|[Continuous Subarray Sum](https://leetcode.com/problems/continuous-subarray-sum/)|[Solution](../master/src/main/java/com/fishercoder/solutions/_523.java) | O(n^2) |O(n) | Medium|
+|523|[Continuous Subarray Sum](https://leetcode.com/problems/continuous-subarray-sum/)|[Solution](../master/src/main/java/com/fishercoder/solutions/_523.java) | O(n) |O(1) | Medium| DP
 |522|[Longest Uncommon Subsequence II](https://leetcode.com/problems/longest-uncommon-subsequence-ii/)|[Solution](../master/src/main/java/com/fishercoder/solutions/_522.java) | O(x*n^2) (x is average length of strings)|O(1) | Medium|
 |521|[Longest Uncommon Subsequence I](https://leetcode.com/problems/longest-uncommon-subsequence-i/)|[Solution](../master/src/main/java/com/fishercoder/solutions/_521.java) | O(max(x,y)) (x and y are length of strings) |O(1) | Easy|
 |520|[Detect Capital](https://leetcode.com/problems/detect-capital/)|[Solution](../master/src/main/java/com/fishercoder/solutions/_520.java) | O(n) |O(1) | Easy| 

src/main/java/com/fishercoder/solutions/_523.java

@@ -1,6 +1,11 @@
 package com.fishercoder.solutions;
 
+import java.util.HashMap;
+import java.util.Map;
+
 /**
+ * 523. Continuous Subarray Sum
+ *
  * Given a list of non-negative numbers and a target integer k,
  * write a function to check if the array has a continuous subarray of size at least 2
  * that sums up to the multiple of k, that is, sums up to n*k where n is also an integer.
@@ -21,7 +26,29 @@
 
  */
 public class _523 {
-    /**TODO: could be optimized to O(n) time and O(k) space, reference: https://discuss.leetcode.com/topic/80793/java-o-n-time-o-k-space*/
+
+    public boolean checkSubarraySumOnTimeO1Space(int[] nums, int k) {
+        Map<Integer, Integer> map = new HashMap<>();
+        map.put(0, -1);
+        int sum = 0;
+        for (int i = 0; i < nums.length; i++) {
+            sum += nums[i];
+            if (k != 0) {
+                /**Because if k == 0, sum %= k will throw ArithmeticException.*/
+                sum %= k;
+            }
+            Integer prev = map.get(sum);
+            if (prev != null) {
+                if (i - prev > 1) {
+                    /**This makes sure that it has length at least 2*/
+                    return true;
+                }
+            } else {
+                map.put(sum, i);
+            }
+        }
+        return false;
+    }
 
     public boolean checkSubarraySum(int[] nums, int k) {
         if (nums == null || nums.length == 0) return false;