PermutationCheck_Solution1.js

function isPermutation(strA, strB){
	var isValid = true;
	
	//First validity check
	if (!strA || !strB || strA.length !== strB.length){
		return !isValid;
	}
	
	//Get the map of characters and their occurences in strA
	var charsMap = getMap(strA);
	
	//Check strB characters and occurences on the computed map.
	for (var i = 0; i < strB.length; i++){
		var charB = strB[i];
		
		if (charsMap[charB] !== undefined && charsMap[charB] >= 1){
			charsMap[charB]--; // If current character does appear in A and B, decrement the occurence counter.
		}
		else{
			return !isValid;
		}
	}
	
	return isValid;
}

function getMap(str){
	var map = {};
	
	for (var i = 0; i < str.length; i++){
		var currChar = str[i];
		
		//If current character hasn't been mapped, map it
		if (map[currChar] === undefined){
			map[currChar] = 1;
		}
		else{
			map[currChar]++; //Else increment the occurrence counter.
		}
	}
	
	return map;
}