using stack to track the middle element in 132 pattern

Author: Arjiit

stack/find132pattern.java

@@ -0,0 +1,25 @@
+class Solution {
+// O(N) time and O(N) space
+    public boolean find132pattern(int[] nums) {
+        if (nums == null || nums.length < 3) return false;
+        
+        int[] minArray = new int[nums.length];
+        minArray[0] = nums[0];
+        for (int i=1; i<nums.length; i++) {
+            minArray[i] = Math.min(nums[i], minArray[i-1]);
+        }
+        Stack<Integer> kStack = new Stack<>(); // stack to keep the middle element 2
+        kStack.push(nums[nums.length - 1]);
+        
+        for(int j=nums.length - 1; j>=0; j--) {
+            while (!kStack.isEmpty() && kStack.peek() <= minArray[j]) {
+                kStack.pop(); // since k(2) should be greater than j(1)
+            }
+            if (!kStack.isEmpty() && kStack.peek() < nums[j]) {
+                return true;
+            }
+            kStack.push(nums[j]);
+        }
+        return false;
+    }
+}