algorithms/solutions/data-structures/stack/PostFixEval.js at master · tomcostello/algorithms · GitHub

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import Stack from './Stack';


export const evalPostFix = arr => {
    let stack = new Stack();

    for (var i = 0; i < arr.length; i++) {
	if (typeof arr[i] == 'number'){
	    stack.push(arr[i]);
	}
	if (arr[i] === '-'){
	    let arg1 = stack.pop();
	    let arg2 = stack.pop();
	    if (arg1 && arg2)stack.push(arg1-arg2); else return null;
	}
	if (arr[i] === '+'){
	    let arg1 = stack.pop();
	    let arg2 = stack.pop();
	    if (arg1 && arg2)stack.push(arg1+arg2); else return null;
	}
	if (arr[i] === '*'){
	    let arg1 = stack.pop();
	    let arg2 = stack.pop();
	    if (arg1 && arg2)stack.push(arg1*arg2); else return null;
	}
	if (arr[i] === '/'){
	    let arg1 = stack.pop();
	    let arg2 = stack.pop();
	    if (arg1 && arg2)stack.push(arg1/arg2); else return null;
	}
    }
    let res = stack.pop();

    if (stack.isEmpty() && res ) return res;
    else return null;

}