diff --git a/group02/812350401/.gitignore b/group02/812350401/.gitignore
index 8cd204f114..8380fddb34 100644
--- a/group02/812350401/.gitignore
+++ b/group02/812350401/.gitignore
@@ -4,3 +4,4 @@ src/main/java/train/
 !src/main/resources/**/*.class
 !src/main/resources/**/*.xml
 src/main/resources/**/*.png
+src/main/java/assignments
diff --git a/group02/812350401/pom.xml b/group02/812350401/pom.xml
index 6ad5aa08db..596d874817 100644
--- a/group02/812350401/pom.xml
+++ b/group02/812350401/pom.xml
@@ -50,6 +50,13 @@
             <artifactId>commons-lang3</artifactId>
             <version>3.4</version>
         </dependency>
+        <!-- https://mvnrepository.com/artifact/io.reactivex.rxjava2/rxjava -->
+        <dependency>
+            <groupId>io.reactivex.rxjava2</groupId>
+            <artifactId>rxjava</artifactId>
+            <version>2.0.8</version>
+        </dependency>
+
     </dependencies>
 
 
diff --git a/group02/812350401/src/main/java/com/github/miniyk2012/coding2017/basic/stack/expr/InfixExpr.java b/group02/812350401/src/main/java/com/github/miniyk2012/coding2017/basic/stack/expr/InfixExpr.java
new file mode 100644
index 0000000000..92727ce984
--- /dev/null
+++ b/group02/812350401/src/main/java/com/github/miniyk2012/coding2017/basic/stack/expr/InfixExpr.java
@@ -0,0 +1,134 @@
+package com.github.miniyk2012.coding2017.basic.stack.expr;
+
+import java.util.LinkedList;
+import java.util.List;
+import java.util.Stack;
+
+public class InfixExpr {
+	private String expr = null;
+    enum Operator {
+        PLUS("+", 1), SUB("-", 1), MUL("*", 2), DIV("/", 2);  // 优先级越大，说明应该先算
+        private String value;
+        private int priority;
+        Operator(String value, int prioriy) {
+            this.value = value;
+            this.priority = prioriy;
+        }
+        public static Operator parseOperator(String value) {
+            for (Operator c : Operator.values()) {
+                if (c.value.equals(value)) {
+                    return c;
+                }
+            }
+            throw new RuntimeException("运算符不存在");
+        }
+    }
+
+	public InfixExpr(String expr) {
+		this.expr = expr;
+	}
+	private static Token<Float> evaluate(Token<Operator> operator, Token<Float> number1, Token<Float> number2) {
+        float ret;
+        switch (operator.value) {
+            case PLUS:
+                ret = number1.getValue() + number2.getValue();
+                break;
+            case SUB:
+                ret = number1.getValue() - number2.getValue();
+                break;
+            case MUL:
+                ret = number1.getValue() * number2.getValue();
+                break;
+            case DIV:
+                ret = number1.getValue() / number2.getValue();
+                break;
+            default:
+                throw new RuntimeException("运算符不存在");
+        }
+        return new Token(ret, true);
+    }
+
+
+	public float evaluate() {
+        List<Token> tokens= Token.parse(expr);
+        Stack<Token<Float>> numberStack = new Stack<>();
+        Stack<Token<Operator>> operatorStack = new Stack<>();
+        for (Token t : tokens) {
+            if (t.isNumber()) {
+                numberStack.push(t);
+            } else {
+                if (operatorStack.isEmpty()) {
+                    operatorStack.push(t);
+                } else {
+                    Token<Operator> preT = operatorStack.peek();
+                    if ((preT.value).priority >= ((Operator)t.value).priority) {
+                        Token number2 = numberStack.pop();
+                        Token number1 = numberStack.pop();
+                        operatorStack.pop();
+                        numberStack.push(evaluate(preT, number1, number2));
+                    }
+                    operatorStack.push(t);
+
+                }
+            }
+        }
+        while (!operatorStack.isEmpty()) {
+            Token number2 = numberStack.pop();
+            Token number1 = numberStack.pop();
+            Token operator = operatorStack.pop();
+            numberStack.push(evaluate(operator, number1, number2));
+        }
+		return numberStack.pop().getValue();
+	}
+
+
+    private static class Token<T> {
+	    private boolean number;  // 是数字/操作符
+	    private T value;
+	    public boolean isNumber() {
+	        return number;
+        }
+        public T getValue() {
+	        return value;
+        }
+
+        public Token(T value, boolean number) {
+            this.value = value;
+            this.number = number;
+        }
+        public static List<Token> parse(String expr) {
+	        int i = 0;
+	        int size = expr.length();
+            String current;
+            List<Token> tokens = new LinkedList<>();
+            String value = "";
+            while (i < size) {
+                current = expr.substring(i,i+1);
+                if (isOperator(current)) {
+                    tokens.add(new Token(Float.parseFloat(value), true));
+                    value = "";
+                    tokens.add(new Token(Operator.parseOperator(current), false));
+                } else {
+                    value += current;
+                }
+                i++;
+            }
+            tokens.add(new Token(Float.parseFloat(value), true));
+            return tokens;
+        }
+
+        private static boolean isOperator(String value) {
+            for (Operator c : Operator.values()) {
+                if (c.value.equals(value)) {
+                    return true;
+                }
+            }
+            return false;
+        }
+    }
+
+    public static void main(String[] args) {
+
+
+    }
+}
diff --git a/group02/812350401/src/main/java/com/github/miniyk2012/coding2017/basic/stack/expr/InfixExprTest.java b/group02/812350401/src/main/java/com/github/miniyk2012/coding2017/basic/stack/expr/InfixExprTest.java
new file mode 100644
index 0000000000..706aaea86e
--- /dev/null
+++ b/group02/812350401/src/main/java/com/github/miniyk2012/coding2017/basic/stack/expr/InfixExprTest.java
@@ -0,0 +1,52 @@
+package com.github.miniyk2012.coding2017.basic.stack.expr;
+
+import org.junit.After;
+import org.junit.Assert;
+import org.junit.Before;
+import org.junit.Test;
+
+
+public class InfixExprTest {
+
+	@Before
+	public void setUp() throws Exception {
+	}
+
+	@After
+	public void tearDown() throws Exception {
+	}
+
+	@Test
+	public void testEvaluate() {
+		//InfixExpr expr = new InfixExpr("300*20+12*5-20/4");
+		{
+			InfixExpr expr = new InfixExpr("2+3");
+			Assert.assertEquals(5.0, expr.evaluate(), 0.001f);
+		}
+		{
+			InfixExpr expr = new InfixExpr("2+3*4+5");
+			Assert.assertEquals(19.0, expr.evaluate(), 0.001f);
+		}
+		{
+			InfixExpr expr = new InfixExpr("3*20+12*5-40/2");
+			Assert.assertEquals(100.0, expr.evaluate(), 0.001f);
+		}
+		
+		{
+			InfixExpr expr = new InfixExpr("3*20/2");
+			Assert.assertEquals(30, expr.evaluate(), 0.001f);
+		}
+		
+		{
+			InfixExpr expr = new InfixExpr("20/2*3");
+			Assert.assertEquals(30, expr.evaluate(), 0.001f);
+		}
+		
+		{
+			InfixExpr expr = new InfixExpr("10-30+50");
+			Assert.assertEquals(30, expr.evaluate(), 0.001f);
+		}
+		
+	}
+
+}