MergeSort in Javascript

MergeSort

/**
 * 合并排序--升序
 * 算法效率: O(arrA.length + arrB.length)
 * 条件为：arrA和arrB均是升序排列数组
 * @Returns 合并后数组
 */

function merge_asc(arrA, arrB) {
    if (!arrA instanceof Array || !arrB instanceof  Array) {
        console.log('invalid array');
        return;
    }
    var arrC = new Array(arrA.length + arrB.length);
    var i = 0, j = 0, k = 0;
    while (i < arrA.length && j < arrB.length) {
        if (arrA[i] <= arrB[j]) {
            arrC[k] = arrA[i];
            ++i;
            ++k;
        } else {
            arrC[k] = arrB[j];
            ++j;
            ++k;
        }
    }
    while (i < arrA.length) {
        arrC[k] = arrA[i];
        ++i;
        ++k;
    }
    while (j < arrB.length) {
        arrC[k] = arrB[j];
        ++j;
        ++k;
    }
    return arrC;
}

var a = [1,2,7,9,4];
var b = [3,5,6,8];

结果不正确