Java源碼分析(1):二分尋找 + 迴圈遞迴實現

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原始碼

源碼地址

    public static int binarySearch(int[] a, int key) {        return binarySearch0(a, 0, a.length, key);    }    public static int binarySearch(int[] a, int fromIndex, int toIndex,                                   int key) {        rangeCheck(a.length, fromIndex, toIndex);        return binarySearch0(a, fromIndex, toIndex, key);    }    // Like public version, but without range checks.    private static int binarySearch0(int[] a, int fromIndex, int toIndex,                                     int key) {        int low = fromIndex;        int high = toIndex - 1;        while (low <= high) {            int mid = (low + high) >>> 1;            int midVal = a[mid];            if (midVal < key)                low = mid + 1;            else if (midVal > key)                high = mid - 1;            else                return mid; // key found        }        return -(low + 1);  // key not found.    }

思考

為啥是mid + 1 ,mid - 1就一個下標感覺沒差啊。

回答：調試之後再回想，發現沒有什麼差別，最終收攏到 low == high 的時候都能算出來，不會錯過。

自己手打的代碼

public class Source1_BinarySearch {    public static int binarySearch(int[] a, int fromIndex, int toIndex, int key) {        int low = fromIndex;        int high = toIndex - 1;        while (low <= high) {            int mid = (low + high) >> 1;            int midValue = a[mid];            // 小於 lo 指向中間，大於hi指向中間,等於直接返回            if (midValue < key)                low = mid + 1;            else if (midValue > key)                high = mid - 1;            else                return mid;        }        return -1;    }    public static int binarySearch_Recursive(int[] a, int fromIndex, int toIndex, int key) {        int low = fromIndex;        int high = toIndex - 1;        int result = -1;        if (low <= high) {            int mid = (low + high) >>> 1;            int midValue = a[mid];            if(midValue < key)                result = binarySearch_Recursive(a,mid +1,high + 1,key);            else if  (midValue > key)                result = binarySearch_Recursive(a,low,mid + 1 -1,key);            else                return mid;        }        return result;    }    public static void main(String[] args) {        int[] a = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10};        int index = binarySearch_Recursive(a, 0, a.length, 11);        System.out.println(index);    }}

Java源碼分析(1):二分尋找 + 迴圈遞迴實現