Create 0907-sum-of-subarray-minimums.java

Author: sujal-goswami

java/0907-sum-of-subarray-minimums.java

@@ -0,0 +1,33 @@
+/*-------------------------------
+  Time Complexity: O(n)
+  Space Complexity: O(n)
+-------------------------------*/
+class Solution {
+    public int sumSubarrayMins(int[] arr) {
+        int MOD = 1000000007;
+        int res = 0;
+        int[] extendedArr = new int[arr.length + 2];
+
+        for (int i = 0; i < arr.length; i++) {
+            extendedArr[i + 1] = arr[i];
+        }
+        extendedArr[0] = Integer.MIN_VALUE;
+        extendedArr[extendedArr.length - 1] = Integer.MIN_VALUE;
+
+        Stack<int[]> stack = new Stack<>(); // (index, num)
+
+        for (int i = 0; i < extendedArr.length; i++) {
+            while (!stack.isEmpty() && extendedArr[i] < stack.peek()[1]) {
+                int[] popped = stack.pop();
+                int j = popped[0];
+                int m = popped[1];
+                int left = j - (stack.isEmpty() ? -1 : stack.peek()[0]);
+                int right = i - j;
+                res = (int) ((res + (long) m * left * right) % MOD);
+            }
+            stack.push(new int[] { i, extendedArr[i] });
+        }
+
+        return res;
+    }
+}