Seventy Commit: Add Remove K Digits problem to Monotonic Stack section

Author: AAdewunmi

src/Monotonic_Stack/Problem_6_Remove_K_Digits.java

@@ -0,0 +1,30 @@
+package Monotonic_Stack;
+
+// Problem Statement: Remove K Digits (hard)
+// LeetCode Question: 402. Remove K Digits
+
+import java.util.Stack;
+
+public class Problem_6_Remove_K_Digits {
+    public String removeKdigits(String num, int k) {
+        Stack<Character> stack = new Stack<>();
+        for (char digit : num.toCharArray()){
+            while(k > 0 && !stack.isEmpty() && stack.peek() > digit){
+                stack.pop();
+                k--;
+            }
+            stack.push(digit);
+        }
+        for (int i = 0; i < k; i++) {
+            stack.pop();
+        }
+        StringBuilder sb = new StringBuilder();
+        while (!stack.isEmpty()) {
+            sb.insert(0, stack.pop());
+        }
+        while (sb.length() > 1 && sb.charAt(0) == '0') {
+            sb.deleteCharAt(0);
+        }
+        return (sb.length() == 0) ? "0" : sb.toString();
+    }
+}