Add monotone stack example

MonotoneStack/MonotoneStack.java

@@ -0,0 +1,26 @@
+public class MonotoneStack{
+
+/*
+* Given an array (i.e. [5, 2, 3, 1, 4]), for each element in the array, find minimum steps to move to find next element larger than itself.
+* If such next element doesn't exist, step = -1;
+* Using above array as an example, we should return [-1, 1, 2, 1, -1]
+*/
+  public static int[] monotonousStack(int[] nums){
+    int[] res = new int[nums.length];
+    Stack<Integer> stack = new Stack<>();
+    // maintain a stack ensuring ascending order from top to bottom
+    for(int i = 0; i < nums.length; ++i){
+      while(!stack.isEmpty() && nums[i] > nums[stack.peek()]){
+        int index = stack.pop();
+        res[index] = i - index;
+      }
+      stack.push(i);
+    }
+    while(!stack.isEmpty()){
+      int index = stack.pop();
+      res[index] = -1;
+    }
+    // as each element is pushed and popped once, the time complexity is O(n)
+    return res;
+  }
+}