js中常见的排序算法

1.冒泡排序

算法描述：相邻两个元素之间进行比较，如果前面的元素大于后面的元素，则进行交换，这样一趟下来，就将整个数组里的最大值排在了最后面，然后再排前面的数

最坏情况如下：

function bubbleSort(arr) {
    var len = arr.length;
    var temp;

    for (var i = 0; i < len; i++) {
        for (var j = 0; j < len-i-1; j++) {
            if (arr[j] > arr[j+1]) {
                temp = arr[j];
                arr[j] = arr[j+1];
                arr[j+1] = temp; 
            }
        }
    }
}

2.选择排序

算法描述：在当前值之后的所有数里，挑出最小的和当前值进行比较，如果小于当前值，则交换位置

function selectSort(arr) {
    var len = arr.length;
    var minIndex, temp;

    for (var i = 0; i < len-1; i++) {
        minIndex = i;

        for (var j = i + 1; j < len; j++) {
            if (arr[j] < arr[minIndex]) {     
                minIndex = j;                 
            }
        }

        temp = arr[i];
        arr[i] = arr[minIndex];
        arr[minIndex] = temp;
    }

    return arr;
}

3.插入排序

算法描述：将当前值与前面的数由后向前比较，如果当前值小于比较的值，则将当前值插入比较值的后面（类似于扑克牌的整理）

function insertSort(arr) {
    var len = arr.length;
    var preIndex, current;

    for (var i = 1; i < len; i++) {
        preIndex = i -1;
        current = arr[i];

        while (preIndex>=0 && arr[preIndex] > current) {
            arr[preIndex+1] = arr[preIndex];
            preIndex--;
        }

        arr[preIndex+1] = current;
    }
    return arr;
}

4.希尔排序

function shellSort(arr) {
        var len = arr.length;
        var fraction;

        for (fraction = Math.floor(len/2); fraction > 0; fraction = Math.floor(fraction/2)) {
            for (var i = fraction; i < len; i++) {
                for (var j = i-fraction; j >= 0 && arr[j] > arr[j+fraction]; j -= fraction) {
                    temp = arr[j];
                    arr[j] = arr[j+fraction];
                    arr[j+fraction] = temp;
                }
            }
        }

        console.log(arr);
    }

5.归并排序

function mergeSort(arr) {
    var len = arr.length;

    if (len < 2) {
        return arr;
    }

    var middle= Math.floor(len / 2);
    var left = arr.slice(0, middle);
    var right = arr.slice(middle);

    return merge(mergeSort(left), mergeSort(right));
}

function merge(left, right) {
    var result = [];

    while (left.length > 0 && right.length > 0) {
        if (left[0] <= right[0]) {
            result.push(left.shift());
        } else {
            result.push(right.shift());
        }
    }

    while (left.length) {
        result.push(left.shift());
    }

    while (right.length) {
        result.push(right.shift());
    }

    return result;
}

var arr = [3, 1, 2, 4];
console.log(mergeSort(arr));

6.快速排序

function quickSort(arr, left, right) {
    var len = arr.length,
        partitionIndex,
        left = typeof left != 'number' ? 0 : left,
        right = typeof right != 'number' ? len - 1 : right;

    if (left < right) {
        partitionIndex = partition(arr, left, right);
        quickSort(arr, left, partitionIndex-1);
        quickSort(arr, partitionIndex+1, right);
    }
    return arr;
}

function partition(arr, left ,right) {     //分区操作
    var pivot = left,                      //设定基准值（pivot）
        index = pivot + 1;
    for (var i = index; i <= right; i++) {
        if (arr[i] < arr[pivot]) {
            swap(arr, i, index);
            index++;
        }        
    }
    swap(arr, pivot, index - 1);
    return index-1;
}

function swap(arr, i, j) {
    var temp = arr[i];
    arr[i] = arr[j];
    arr[j] = temp;
}

var arr = [5, 4, 3, 2, 1];
console.log(quickSort(arr));   // 1, 2, 3, 4, 5

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