Create Basic Calculator 2.java

Author: Prakhar Agarwal

Basic Calculator 2.java

@@ -0,0 +1,71 @@
+import java.util.ArrayList;
+import java.util.List;
+import java.util.Stack;
+
+class Solution {
+    public int calculate(String s) {
+        return solvePostFix(postFix(s));
+    }
+
+    private int solvePostFix(List<Object> list) {
+        Stack<Integer> st = new Stack();
+        for (Object o : list) {
+            if (o instanceof Integer) {
+                st.push((Integer) o);
+            } else {
+                char ch = (Character) o;
+                int b = st.pop();
+                int a = st.pop();
+                if (ch == '+') {
+                    st.push(a + b);
+                } else if (ch == '-') {
+                    st.push(a - b);
+                } else if (ch == '*') {
+                    st.push(a * b);
+                } else if (ch == '/') {
+                    st.push(a / b);
+                }
+            }
+        }
+        return st.pop();
+    }
+
+    private List<Object> postFix(String s) {
+        List<Object> list = new ArrayList();
+        Stack<Character> st = new Stack();
+        int num = 0;
+        for (char ch : s.toCharArray()) {
+            if (ch == ' ')
+                continue;
+            if (Character.isDigit(ch)) {
+                num = num * 10 + (ch - '0');
+            } else {
+                list.add(num);
+                num = 0;
+                while (!st.isEmpty() && prec(st.peek()) >= prec(ch)) {
+                    list.add(st.pop());
+                }
+                st.push(ch);
+            }
+        }
+        list.add(num);
+        while (!st.isEmpty()) {
+            list.add(st.pop());
+        }
+        return list;
+    }
+
+    private int prec(char ch) {
+        switch (ch) {
+            case '+':
+            case '-':
+                return 1;
+            case '*':
+            case '/':
+                return 2;
+            case '^':
+                return 3;
+        }
+        return -1;
+    }
+}