Stack

Stack

Author: dandanhub

.classpath

@@ -1,6 +1,7 @@
 <?xml version="1.0" encoding="UTF-8"?>
 <classpath>
 	<classpathentry kind="src" path="LinkedList"/>
+	<classpathentry kind="src" path="Stack"/>
 	<classpathentry kind="con" path="org.eclipse.jdt.launching.JRE_CONTAINER"/>
 	<classpathentry kind="output" path="bin"/>
 </classpath>

Queue/README.md

@@ -1,4 +1,4 @@
-The codes here briefly summarizes my work on queue.
+The java codes here basically summarize my work on Queue. The codes are driven by CMU course 08-722 Data Structures for Application Programmers and some leetcode problems related about Queue.
 
 # MovingAverage.java
 ## 346. Moving Average from Data Stream

LinkedList/SnakeGame.java Queue/SnakeGame.java

@@ -130,6 +130,7 @@ public static void main(String[] args) {
         System.out.println(obj.move("U"));
         System.out.println(obj.move("R"));
         System.out.println(obj.move("D"));
+       
     }
 }
 

Stack/README.md

@@ -0,0 +1,24 @@
+The java codes here basically summarize my work on Stack. The codes are driven by CMU course 08-722 Data Structures for Application Programmers and some LeetCode problems related about Stack.
+
+# Solution.java
+## 224. Basic Calculator
+Implement a basic calculator to evaluate a simple expression string.
+
+The expression string may contain open ( and closing parentheses ), the plus + or minus sign -, non-negative integers and empty spaces .
+
+You may assume that the given expression is always valid.
+
+Some examples:
+~~~~
+"1 + 1" = 2
+" 2-1 + 2 " = 3
+"(1+(4+5+2)-3)+(6+8)" = 23
+~~~~
+Note: Do not use the eval built-in library function.
+
+#### Solution
+1. First remove spaces of the string s.
+2. Use stack to handle parentheses, pop the stack when encountering ')' until popping out '('.
+3. Evaluate the string without parentheses using calculateHelper.
+
+Be careful with '-', for example, "2-(5-6)".

Stack/Solution.java

@@ -0,0 +1,89 @@
+import java.util.Stack;
+
+public class Solution {
+    public int calculate(String s) {
+        if (s == null || s.length() == 0) {
+            return 0; // how to handle the corner case?
+        }
+        
+        s = s.replaceAll("\\s","");
+        if (!s.contains("(") && !s.contains(")")) {
+            return calculateHelper(s);
+        }
+        
+        int pos = 0;
+        int len = s.length();
+        Stack<String> stack = new Stack<String>();
+        while (pos < len) {
+            if (s.charAt(pos) == ')') {
+                StringBuilder sb = new StringBuilder();
+                String prev = stack.pop();
+                while (!prev.equals("(")) {
+                    sb.insert(0, prev);
+                    prev = stack.pop();
+                }
+                int ans = calculateHelper(sb.toString());
+                stack.push(Integer.toString(ans));
+            }
+            else {
+                stack.push(String.valueOf(s.charAt(pos)));
+            }
+            pos++;
+        }
+        
+        StringBuilder sb = new StringBuilder();
+        while (!stack.isEmpty()) {
+            String str = stack.pop();
+            sb.insert(0, str);
+        }
+        
+        return calculateHelper(sb.toString());
+    }
+    
+    private int calculateHelper(String s) {
+        int index = 0;
+        int len = s.length();
+        int ans = 0;
+        char operator = '+';
+        while (index < len) {
+            StringBuilder sb = new StringBuilder();
+            if (s.charAt(index) == '-') {
+                operator = operator == '+' ? '-' : '+';
+                index++;
+            }
+            while (index < len && s.charAt(index) != '+' && s.charAt(index) != '-') {
+                sb.append(s.charAt(index++));
+            }
+            
+            if (operator == '+') {
+                ans += Integer.parseInt(sb.toString());
+            }
+            else {
+                ans -= Integer.parseInt(sb.toString());
+            }
+            if (index < len) {
+                operator = s.charAt(index++);
+            }
+        }
+        return ans;
+    }
+    
+    public static void main(String[] args) {
+        Solution s = new Solution();
+        String x = "2-(5-6)";
+        System.out.println(x);
+        System.out.println(s.calculate(x));
+    }
+    
+    /*
+    test case:
+    ""
+    "   "
+    "0"
+    "1 + 1"
+    " 2-1 + 2 "
+    "(1+(4+5+2)-3)+(6+8)"
+    "(1+(4+5+2)-3)+((6 + 8) + (6+8))"
+    "2-(5-6)"
+    */
+}