检索算法——二分查找

    function insertionSort(arr) {
        var temp, inner;
        for ( var outer = 1; outer < arr.length; ++outer) {
            temp = arr[outer];
            //选中一个值作为临时值，使得其前面的数依次与它进行比较
            inner = outer;
            while (inner > 0 && (arr[inner - 1] >= temp)) {
                arr[inner] = arr[inner - 1];
                --inner;
            }
            arr[inner] = temp;
        }
    }
    function prints(arr) {
        for ( var i = 0; i < arr.length; i++) {
            document.write(arr[i] + " ");
            if (i > 0 && i % 10 == 0) {
                document.write("<br />");
            }
        }

    }
    function binSearch(arr, data) {
        var upperBound = arr.length - 1;
        var lowerBound = 0;
        while (lowerBound <= upperBound) {
            var mid = Math.floor((upperBound + lowerBound) / 2);
            if (arr[mid] < data) {
                lowerBound = mid + 1;
            } else if (arr[mid] > data) {
                upperBound = mid - 1;
            } else {
                return mid;
            }
        }
        return -1;
    }
    var nums = [];
    for ( var i = 0; i < 100; ++i) {
        nums[i] = Math.floor(Math.random() * 101);
    }
    insertionSort(nums);
    prints(nums);
    var val = 45;
    var retVal = binSearch(nums, val);
    if (retVal >= 0) {
        document.write(" 已找到 " + val + " ， 所在位置为： " + retVal);
    } else {
        document.write(val + " 没有出现在这个数组中。 ");
    }

    function count(arr, data) {
        //为什么需要count()———>在二分查找中要是有三个要查找的值，那么binSearch()方法仅会返回中间值所在的位置
        var count = 0;
        var position = binSearch(arr, data);
        if (position > -1) {//判断是否在数组中找到data
            ++count;
            for ( var i = position - 1; i > 0; --i) {//向下查找
                if (arr[i] == data) {
                    ++count;
                } else {
                    break;
                }
            }
            for ( var i = position + 1; i < arr.length; ++i) {//向下查找
                if (arr[i] == data) {
                    ++count;
                } else {
                    break;
                }
            }
        }
        return count;
    }
    alert(count(nums, val));