infix to postfix, without parenthesis

infixToPostfix.js

// a + b * c + d

// a b c * + d +
// a + b * c + d

// a b c * + d +



function infixToPostfix(input) {
	let stack = [], answer = [], char;
	const operands = {
		'+' : 1,
		'-' : 1,
		"*" : 2,
		"/" : 2,
	};

	for(let i = 0; i < input.length; i++ ) {
		char = input.charAt(i);
		let currentPrecedence = operands[char];
		
		if (currentPrecedence) {
			
			let peek = operands[stack[stack.length - 1]];
			
			// pop until the peek is smaller
			while(peek >= currentPrecedence) {
				answer.push(stack.pop());
				peek = operands[stack[stack.length - 1]];
			}
			
			stack.push(char);

		} else { // not operand, push to answer
			answer.push(char);
		}
	}

	while(stack.length > 0) {
		answer.push(stack.pop())
	}

	return answer.join('');
}

function executePostfix(str) {
	let stack = [], char, operand1, operand2, tempOperand;
	let operators = [ '+', '-', '*', '/'];

	for(let i = 0; i < str.length; i++) {
		char = str.charAt(i);
		if (operators.indexOf(char) >= 0) {
			// operate
			operand2 = stack.pop();
			operand1 = stack.pop();

			tempOperand = eval(operand1+char+operand2);
			stack.push(tempOperand);

		} else {
			stack.push(char);
		}
	}
	return stack.pop();
}
// let p = infixToPostfix('a+b*c+d');

let p = infixToPostfix('2+3*3+7');
let answer = executePostfix(p);

console.log(p, answer);

233*+7+ 18
[Finished in 0.1s]