Create 918-maximum-sum-circular-subarray.md

Author: everthis

detailed/918-maximum-sum-circular-subarray.md

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+![alt text](https://github.com/everthis/leetcode-js/blob/master/images/maximum-sum-circular-subarray.png "maximum-sum-circular-subarray")
+
+```js
+/**
+ * @param {number[]} A
+ * @return {number}
+ */
+const maxSubarraySumCircular = function(A) {
+  let minSum = Infinity, sum = 0, maxSum = -Infinity, curMax = 0, curMin = 0
+  for(let a of A) {
+    sum += a
+    curMax = Math.max(curMax + a, a);
+    maxSum = Math.max(maxSum, curMax);
+    curMin = Math.min(curMin + a, a);
+    minSum = Math.min(minSum, curMin);
+  }
+  return  maxSum > 0 ? Math.max(maxSum, sum - minSum) : maxSum;
+};
+```