Create infix-to-postfix.md

Solved the issue (codemistic#451 )

Author: tanuj1811

Java/Stack/infix-to-postfix.md

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+# Infix Notation
+
+Infix is the day to day notation that we use of format A + B type. The general form can be classified as (a op b) where a and b are operands(variables) and op is Operator.
+
+# Postfix Notation
+
+Postfix is notation that compiler uses/converts to while reading left to right and is of format AB+ type. The general form can be classified as (ab op) where a and b are operands(variables) and op is Operator.
+
+## Code
+
+``` 
+import java.util.Stack;
+
+public class InfixtoPostfix {
+public static void main(String[] args) {
+String exp = "a+b*(c^d-e)^(f+g*h)-i";
+System.out.println(infixToPostfix(exp));
+}
+public static int Precidense(char ch) {
+switch (ch){
+ case '+':
+ case '-':
+ return 1;
+
+ case '*':
+ case '/':
+ return 2;
+
+ case '^':
+ return 3;
+}
+return -1;
+}
+
+public static String infixToPostfix(String exp) {
+String result = "";
+Stack<Character> stack = new Stack<>();
+
+for (char c:exp.toCharArray()) {
+if (Character.isLetterOrDigit(c))
+result += c;
+else if (c == '(')
+stack.push(c);
+else if (c == ')') {
+while (!stack.isEmpty() && stack.peek() != '(')
+result += stack.pop();
+stack.pop();
+}
+else {// an operator is encountered
+while (!stack.isEmpty() && Precidense(c) <= Precidense(stack.peek()))
+result += stack.pop();
+stack.push(c);
+}
+
+}
+while (!stack.isEmpty()){
+if(stack.peek() == '(')
+return "Invalid Expression";
+result += stack.pop();
+}
+return result;
+}
+}
+```