您将得到一个代表数字和操作数表达式的字符串。例如1 + 2 * 3、1-2 + 4。您需要评估字符串或表达式。不遵循BODMAS。如果表达式语法错误，则返回-1。
测试用例：
a)1 + 2 * 3将被评估为9。
b)4-2 + 6 * 3将计算为24。
c)1 ++ 2将被评估为-1(INVALID)。
同样，在字符串可能会出现空格。在这种情况下，我们需要忽略空格。像：-1 * 2 -1等于1。
资料来源：亚马逊访谈问题
强烈建议最小化浏览器，然后自己尝试。
这个想法很简单，从第一个字符开始，然后从左向右遍历，并检查是否有错误，例如两个连续的运算符和操作数。我们还跟踪结果并在遍历表达式时更新结果。
以下是评估给定表达式的程序。
C++
// C++ program to evaluate a given expression
#include
using namespace std;
// A utility function to check if a given character is operand
bool isOperand(char c) { return (c >= '0' && c <= '9'); }
// utility function to find value of and operand
int value(char c) { return (c - '0'); }
// This function evaluates simple expressions. It returns -1 if the
// given expression is invalid.
int evaluate(char *exp)
{
// Base Case: Given expression is empty
if (*exp == '\0') return -1;
// The first character must be an operand, find its value
int res = value(exp[0]);
// Traverse the remaining characters in pairs
for (int i = 1; exp[i]; i += 2)
{
// The next character must be an operator, and
// next to next an operand
char opr = exp[i], opd = exp[i+1];
// If next to next character is not an operand
if (!isOperand(opd)) return -1;
// Update result according to the operator
if (opr == '+') res += value(opd);
else if (opr == '-') res -= value(opd);
else if (opr == '*') res *= value(opd);
else if (opr == '/') res /= value(opd);
// If not a valid operator
else return -1;
}
return res;
}
// Driver program to test above function
int main()
{
char expr1[] = "1+2*5+3";
int res = evaluate(expr1);
(res == -1)? cout << expr1 << " is " << "Invalid\n":
cout << "Value of " << expr1 << " is " << res << endl;
char expr2[] = "1+2*3";
res = evaluate(expr2);
(res == -1)? cout << expr2 << " is " << "Invalid\n":
cout << "Value of " << expr2 << " is " << res << endl;
char expr3[] = "4-2+6*3";
res = evaluate(expr3);
(res == -1)? cout << expr3 << " is " << "Invalid\n":
cout << "Value of " << expr3 << " is " << res << endl;
char expr4[] = "1++2";
res = evaluate(expr4);
(res == -1)? cout << expr4 << " is " << "Invalid\n":
cout << "Value of " << expr4 << " is " << res << endl;
return 0;
} Java
// Java program to evaluate a given expression
class GFG{
// A utility function to check if
// a given character is operand
static boolean isOperand(char c)
{
return (c >= '0' && c <= '9');
}
// utility function to find value of and operand
static int value(char c)
{
return (int)(c - '0');
}
// This function evaluates simple expressions.
// It returns -1 if the given
// expression is invalid.
static int evaluate(String exp)
{
// Base Case: Given expression is empty
if (exp.length() == 0) return -1;
// The first character must be
// an operand, find its value
int res = value(exp.charAt(0));
// Traverse the remaining characters in pairs
for (int i = 1; iPython3
# Python3 program to evaluate a
# given expression
# A utility function to check if
# a given character is operand
def isOperand(c):
return (c >= '0' and c <= '9');
# utility function to find
# value of and operand
def value(c):
return ord(c) - ord('0');
# This function evaluates simple
# expressions. It returns -1 if the
# given expression is invalid.
def evaluate(exp):
len1 = len(exp);
# Base Case: Given expression is empty
if (len1 == 0):
return -1;
# The first character must be
# an operand, find its value
res = value(exp[0]);
# Traverse the remaining
# characters in pairs
for i in range(1,len1,2):
# The next character must be
# an operator, and next to
# next an operand
opr = exp[i];
opd = exp[i + 1];
# If next to next character
# is not an operand
if (isOperand(opd)==False):
return -1;
# Update result according
# to the operator
if (opr == '+'):
res += value(opd);
elif (opr == '-'):
res -= int(value(opd));
elif (opr == '*'):
res *= int(value(opd));
elif (opr == '/'):
res /= int(value(opd));
# If not a valid operator
else:
return -1;
return res;
# Driver Code
expr1 = "1+2*5+3";
res = evaluate(expr1);
print(expr1,"is Invalid") if (res == -1) else print("Value of",expr1,"is",res);
expr2 = "1+2*3";
res = evaluate(expr2);
print(expr2,"is Invalid") if (res == -1) else print("Value of",expr2,"is",res);
expr3 = "4-2+6*3";
res = evaluate(expr3);
print(expr3,"is Invalid") if (res == -1) else print("Value of",expr3,"is",res);
expr4 = "1++2";
res = evaluate(expr4);
print(expr4,"is Invalid") if (res == -1) else print("Value of",expr4,"is",res);
# This code is contributed by mitsC#
// C# program to evaluate a given expression
using System;
class GFG{
// A utility function to check if
// a given character is operand
static bool isOperand(char c) {
return (c >= '0' && c <= '9');
}
// utility function to find value of and operand
static int value(char c) { return (int)(c - '0'); }
// This function evaluates simple
// expressions. It returns -1 if the
// given expression is invalid.
static int evaluate(string exp)
{
// Base Case: Given expression is empty
if (exp.Length == 0) return -1;
// The first character must be
// an operand, find its value
int res = value(exp[0]);
// Traverse the remaining characters in pairs
for (int i = 1; iPHP
= '0' && $c <= '9');
}
// utility function to find
// value of and operand
function value($c)
{
return ($c - '0');
}
// This function evaluates simple
// expressions. It returns -1 if the
// given expression is invalid.
function evaluate($exp)
{
$len = strlen($exp);
// Base Case: Given expression is empty
if ($len == 0) return -1;
// The first character must be
// an operand, find its value
$res = (int)(value($exp[0]));
// Traverse the remaining
// characters in pairs
for ($i = 1; $i < $len; $i += 2)
{
// The next character must be
// an operator, and next to
// next an operand
$opr = $exp[$i];
$opd = $exp[$i + 1];
// If next to next character
// is not an operand
if (!isOperand($opd))
return -1;
// Update result according
// to the operator
if ($opr == '+')
$res += value($opd);
else if ($opr == '-')
$res -= (int)(value($opd));
else if ($opr == '*')
$res *= (int)(value($opd));
else if ($opr == '/')
$res /= (int)(value($opd));
// If not a valid operator
else
return -1;
}
return $res;
}
// Driver Code
$expr1 = "1+2*5+3";
$res = evaluate($expr1);
($res == -1) ? print($expr1." is Invalid\n"):
print("Value of " . $expr1 .
" is " . $res . "\n");
$expr2 = "1+2*3";
$res = evaluate($expr2);
($res == -1) ? print($expr2." is Invalid\n"):
print("Value of " . $expr2 .
" is " . $res . "\n");
$expr3 = "4-2+6*3";
$res = evaluate($expr3);
($res == -1) ? print($expr3." is Invalid\n"):
print("Value of " . $expr3 .
" is " . $res . "\n");
$expr4 = "1++2";
$res = evaluate($expr4);
($res == -1) ? print($expr4." is Invalid\n"):
print("Value of " . $expr4 .
" is " . $res . "\n");
// This code is contributed by mits
?>输出：
Value of 1+2*5+3 is 18
Value of 1+2*3 is 9
Value of 4-2+6*3 is 24
1++2 is Invalid上面的代码不处理空格。我们可以通过首先删除给定字符串的所有空格来处理空格。更好的解决方案是处理单个遍历中的空间。这留作练习。
时间复杂度为O(n)，其中n是给定表达式的长度。