javascript将算法复杂度从O(n^2)做到O(n)

compare the difference of two giving array, return results: 1. elements in both array, 2. elements only in one array.

        // O(m*n + m*n) -> O(2*m*n) -> O(n^2)
        function compareArr1(arr1, arr2){
            Array.prototype.contains = function(v) {
                for (var i = this.length - 1; i >= 0; i--) {
                    if (this[i] == v) {
                        return 1;
                    }
                }
                return 0;
            }

            arr1 = arr1.sort();
            arr2 = arr2.sort();
            var ein12 = [], ein1 = [], ein2 = [];

            for (var i = arr1.length - 1; i >= 0; i--) {
                if(arr2.contains(arr1[i])){
                    ein12.push(arr1[i]);
                }else{
                    ein1.push(arr1[i]);
                }
            }

            for (var i = arr2.length - 1; i >= 0; i--) {
                if(!arr1.contains(arr2[i])){
                    ein2.push(arr2[i]);
                }
            }

            return { 'ein12': ein12, 'ein1': ein1, 'ein2': ein2 }
        }
        // a=[1,2,3,2,3];
        // b=[2,3,4,3,4];
        // compareArr1(a,b);

        // O(m*n + m + n) -> O(n^2)
        function compareArr2(arr1, arr2) {

            function delDup(arr) {
                for (var i = arr.length - 1; i >= 1; i--) {
                    if(arr[i]==arr[i-1]){
                        arr.splice(i,1);
                    }
                }
            }

            arr1 = arr1.sort();
            arr2 = arr2.sort();

            delDup(arr1);
            delDup(arr2);

            var ein12 = [], ein1 = arr1.slice(0, arr1.length), ein2 = arr2.slice(0, arr2.length);

            for (var i = arr1.length - 1; i >= 0; i--) {
                for (var j = arr2.length - 1; j >= 0; j--) {
                    if (arr1[i] == arr2[j]) {
                        ein12.push(arr1[i]);
                        ein1.splice(i, 1);
                        ein2.splice(j, 1);
                        break;
                    }
                }
            }
            return { 'ein12': ein12, 'ein1': ein1, 'ein2': ein2 }
        }
        // a=[1,2,3,2,3];
        // b=[2,3,4,3,4];
        // compareArr2(a,b);

        // O(2m+2n+m+n) -> O(3(m+n)) -> O(n)
        function compareArr3(arr1, arr2){
            function arr2obj(obj,arr) {
                for (var i = arr.length - 1; i >= 0; i--) {
                    obj['_' + arr[i]]=arr[i];
                }
                return obj;
            }
            var obj12 = {}, obj1 = {}, obj2 = {};
            arr2obj(obj1,arr1);
            arr2obj(obj2,arr2);
            arr2obj(obj12,arr1);
            arr2obj(obj12,arr2);

            var ein12 = [], ein1 = [], ein2 = [];

            for(var k in obj12){
                if(obj1[k] && obj2[k])
                {
                    ein12.push(obj12[k]);
                }else if(obj1[k]){
                    ein1.push(obj12[k]);
                }else{
                    ein2.push(obj12[k]);
                }
            }

            return { 'ein12': ein12, 'ein1': ein1, 'ein2': ein2 }
        }
        // a=[1,2,3,2,3];
        // b=[2,3,4,3,4];
        // compareArr3(a,b);

Think more, and you can do better.