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

Author: a93a

kotlin/0907-sum-of-subarray-minimums.kt

@@ -0,0 +1,44 @@
+class Solution {
+    fun sumSubarrayMins(_arr: IntArray): Int {
+        val mod = 1_000_000_007
+        var res = 0L
+        val arr = intArrayOf(Integer.MIN_VALUE) + _arr + intArrayOf(Integer.MIN_VALUE)
+        val stack = Stack<Pair<Int, Int>>()
+
+        for ((i, n) in arr.withIndex()) {
+            while (stack.isNotEmpty() && n < stack.peek().second) {
+                val (j, m) = stack.pop()
+                val left = if (stack.isNotEmpty()) j - stack.peek().first else j + 1
+                val right = i - j
+                res = (res + m.toLong() * left * right) % mod
+            }
+            stack.push(i to n)
+        }
+
+        return res.toInt()
+    }
+}
+
+// Another and just a little different solution, you can also only push indices to the stack
+class Solution {
+    fun sumSubarrayMins(arr: IntArray): Int {
+        val stack = LinkedList<Int>()
+        val mod = 1_000_000_007
+        var res = 0L
+        
+        for (right in 0..arr.size) {  
+            val curVal = if (right < arr.size) arr[right] else 0
+            
+            while (stack.isNotEmpty() && curVal < arr[stack.peekLast()]) {
+                val cur = stack.removeLast()
+                val left = stack.peekLast() ?: -1
+                val noOfSubArrs = (cur.toLong() - left) * (right - cur)
+                res = (res + noOfSubArrs * arr[cur]) % mod
+            }
+            
+            stack.addLast(right)
+        }
+        
+        return res.toInt()
+    }
+}