后缀：如果运算符出现在操作数之后的表达式中，则该表达式称为后缀表达式。形式简单(operand1 operatornd2 运算符)。
例子： AB + CD- *(中缀：(A + B)*(CD))
前缀：如果运算符出现在操作数之前的表达式中，则该表达式称为前缀表达式。形式简单(运算符operond1操作数2)。
例如： * + AB-CD(内缀：(A + B)*(CD))
给定Postfix表达式，请将其转换为Prefix表达式。
将Postfix表达式直接转换为Prefix，而无需经过先将它们转换为Infix然后转换为Prefix的过程，这在计算和更好地理解该表达式方面要好得多(计算机使用Postfix表达式进行评估)。
例子：
Input : Postfix : AB+CD-*
Output : Prefix : *+AB-CD
Explanation : Postfix to Infix : (A+B) * (C-D)
Infix to Prefix : *+AB-CD
Input : Postfix : ABC/-AK/L-*
Output : Prefix : *-A/BC-/AKL
Explanation : Postfix to Infix : ((A-(B/C))*((A/K)-L))
Infix to Prefix : *-A/BC-/AKL
后缀到前缀的算法：
- 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.
string = operator + operand2 + operand1
And push the resultant string back to Stack - Repeat the above steps until end of Prefix expression.
下面是上述想法的实现：
C++
// CPP Program to convert postfix to prefix
#include
using namespace std;
// function to check if character is operator or not
bool isOperator(char x)
{
switch (x) {
case '+':
case '-':
case '/':
case '*':
return true;
}
return false;
}
// Convert postfix to Prefix expression
string postToPre(string post_exp)
{
stack s;
// length of expression
int length = post_exp.size();
// reading from right to left
for (int i = 0; i < length; i++) {
// check if symbol is operator
if (isOperator(post_exp[i])) {
// pop two operands from stack
string op1 = s.top();
s.pop();
string op2 = s.top();
s.pop();
// concat the operands and operator
string temp = post_exp[i] + op2 + op1;
// Push string temp back to stack
s.push(temp);
}
// if symbol is an operand
else {
// push the operand to the stack
s.push(string(1, post_exp[i]));
}
}
string ans = "";
while (!s.empty()) {
ans += s.top();
s.pop();
}
return ans;
}
// Driver Code
int main()
{
string post_exp = "ABC/-AK/L-*";
// Function call
cout << "Prefix : " << postToPre(post_exp);
return 0;
} Java
// Java Program to convert postfix to prefix
import java.util.*;
class GFG {
// function to check if character
// is operator or not
static boolean isOperator(char x)
{
switch (x) {
case '+':
case '-':
case '/':
case '*':
return true;
}
return false;
}
// Convert postfix to Prefix expression
static String postToPre(String post_exp)
{
Stack s = new Stack();
// length of expression
int length = post_exp.length();
// reading from right to left
for (int i = 0; i < length; i++) {
// check if symbol is operator
if (isOperator(post_exp.charAt(i))) {
// pop two operands from stack
String op1 = s.peek();
s.pop();
String op2 = s.peek();
s.pop();
// concat the operands and operator
String temp
= post_exp.charAt(i) + op2 + op1;
// Push String temp back to stack
s.push(temp);
}
// if symbol is an operand
else {
// push the operand to the stack
s.push(post_exp.charAt(i) + "");
}
}
// concatenate all strings in stack and return the
// answer
String ans = "";
for (String i : s)
ans += i;
return ans;
}
// Driver Code
public static void main(String args[])
{
String post_exp = "ABC/-AK/L-*";
// Function call
System.out.println("Prefix : "
+ postToPre(post_exp));
}
}
// This code is contributed by Arnab Kundu Python3
# Python3 Program to convert postfix to prefix
# function to check if
# character is operator or not
def isOperator(x):
if x == "+":
return True
if x == "-":
return True
if x == "/":
return True
if x == "*":
return True
return False
# Convert postfix to Prefix expression
def postToPre(post_exp):
s = []
# length of expression
length = len(post_exp)
# reading from right to left
for i in range(length):
# check if symbol is operator
if (isOperator(post_exp[i])):
# pop two operands from stack
op1 = s[-1]
s.pop()
op2 = s[-1]
s.pop()
# concat the operands and operator
temp = post_exp[i] + op2 + op1
# Push string temp back to stack
s.append(temp)
# if symbol is an operand
else:
# push the operand to the stack
s.append(post_exp[i])
ans = ""
for i in s:
ans += i
return ans
# Driver Code
if __name__ == "__main__":
post_exp = "AB+CD-"
# Function call
print("Prefix : ", postToPre(post_exp))
# This code is contributed by AnkitRai01C#
// C# Program to convert postfix to prefix
using System;
using System.Collections;
class GFG {
// function to check if character
// is operator or not
static Boolean isOperator(char x)
{
switch (x) {
case '+':
case '-':
case '/':
case '*':
return true;
}
return false;
}
// Convert postfix to Prefix expression
static String postToPre(String post_exp)
{
Stack s = new Stack();
// length of expression
int length = post_exp.Length;
// reading from right to left
for (int i = 0; i < length; i++) {
// check if symbol is operator
if (isOperator(post_exp[i])) {
// Pop two operands from stack
String op1 = (String)s.Peek();
s.Pop();
String op2 = (String)s.Peek();
s.Pop();
// concat the operands and operator
String temp = post_exp[i] + op2 + op1;
// Push String temp back to stack
s.Push(temp);
}
// if symbol is an operand
else {
// Push the operand to the stack
s.Push(post_exp[i] + "");
}
}
String ans = "";
while (s.Count > 0)
ans += s.Pop();
return ans;
}
// Driver Code
public static void Main(String[] args)
{
String post_exp = "ABC/-AK/L-*";
// Function call
Console.WriteLine("Prefix : "
+ postToPre(post_exp));
}
}
// This code is contributed by Arnab Kundu输出
Prefix : *-A/BC-/AKL