Postfix to Prefix Conversion
Postfix: An expression is called the postfix expression if the operator appears in the expression after the operands. Simply of the form (operand1 operand2 operator).
Example : AB+CD-* (Infix : (A+B) * (C-D) )
Prefix : An expression is called the prefix expression if the operator appears in the expression before the operands. Simply of the form (operator operand1 operand2).
Example : *+AB-CD (Infix : (A+B) * (C-D) )
Given a Postfix expression, convert it into a Prefix expression.
分析：
- Read the Postfix expression from left to right
- If the symbol is an operand, then push it onto the Stack
- If the symbol is an operator, then pop two operands from the Stack
- Create a string by concatenating the two operands and the operator before them. Like: string = operator + operand2 + operand1, and push the resultant string back to Stack
- Repeat the above steps until end of Prefix expression.
1 class Solution {
2 static boolean isOperator(char x) {
3 switch (x) {
4 case '+':
5 case '-':
6 case '/':
7 case '*':
8 return true;
9 default:
10 return false;
11 }
12 }
13
14 static String postToPre(String exp) {
15 Stack<String> s = new Stack<>();
16 int length = exp.length();
17 for (int i = 0; i < length; i++) {
18 if (isOperator(exp.charAt(i))) {
19 String op1 = s.pop();
20 String op2 = s.pop();
21 String temp = exp.charAt(i) + op2 + op1;
22 s.push(temp);
23 }
24 else {
25 s.push(exp.charAt(i) + "");
26 }
27 }
28 return s.peek();
29 }
30 }
Prefix to Postfix Conversion
Prefix : An expression is called the prefix expression if the operator appears in the expression before the operands. Simply of the form (operator operand1 operand2).
Example : *+AB-CD (Infix : (A+B) * (C-D) )
Postfix: An expression is called the postfix expression if the operator appears in the expression after the operands. Simply of the form (operand1 operand2 operator).
Example : AB+CD-* (Infix : (A+B * (C-D) )
Given a Prefix expression, convert it into a Postfix expression.
分析：
- Read the Prefix expression in reverse order (from right to left)
- If the symbol is an operand, then push it onto the Stack
- If the symbol is an operator, then pop two operands from the Stack
- Create a string by concatenating the two operands and the operator after them.
- string = operand1 + operand2 + operator
- And push the resultant string back to Stack
- Repeat the above steps until end of Prefix expression.
1 class Solution {
2 boolean isOperator(char x) {
3 switch (x) {
4 case '+':
5 case '-':
6 case '/':
7 case '*':
8 return true;
9 default:
10 return false;
11 }
12 }
13
14 String preToPost(String pre_exp) {
15 Stack<String> s = new Stack<>();
16 int length = pre_exp.length();
17 for (int i = length - 1; i >= 0; i--) {
18 if (isOperator(pre_exp.charAt(i))) {
19 String op1 = s.pop();
20 String op2 = s.pop();
21 String temp = op1 + op2 + pre_exp.charAt(i);
22 s.push(temp);
23 } else {
24 s.push(pre_exp.charAt(i) + "");
25 }
26 }
27 return s.peek();
28 }
29 }