《剑指offer》第五十三题（数组中数值和下标相等的元素）

// 面试题53（三）：数组中数值和下标相等的元素
// 题目：假设一个单调递增的数组里的每个元素都是整数并且是唯一的。请编程实
// 现一个函数找出数组中任意一个数值等于其下标的元素。例如，在数组{-3, -1,
// 1, 3, 5}中，数字3和它的下标相等。

//思想：因为这里也是递增的，如果中间点的值大于下标，那后面所有值都会大于相应下标，同理中间点小于下标，前面都会小于下标
#include <iostream>

int GetNumberSameAsIndex(const int* numbers, int length)
{
    if (numbers == nullptr || length <= 0)
        return -1;

    int left = 0;
    int right = length - 1;
    while (left <= right)
    {
        int middle = left + ((right - left) >> 1);
        if (numbers[middle] == middle)//第一种情况，中间点直接中了
            return middle;

        if (numbers[middle] > middle)//第二种情况，中间点值大于坐标，向左区间找
            right = middle - 1;
        else//第三种情况，中间点值小于坐标，向右区间找
            left = middle + 1;
    }

    return -1;
}

// ====================测试代码====================
void Test(const char* testName, int numbers[], int length, int expected)
{
    if (GetNumberSameAsIndex(numbers, length) == expected)
        printf("%s passed.
", testName);
    else
        printf("%s FAILED.
", testName);
}

void Test1()
{
    int numbers[] = { -3, -1, 1, 3, 5 };
    int expected = 3;
    Test("Test1", numbers, sizeof(numbers) / sizeof(int), expected);
}

void Test2()
{
    int numbers[] = { 0, 1, 3, 5, 6 };
    int expected = 0;
    Test("Test2", numbers, sizeof(numbers) / sizeof(int), expected);
}

void Test3()
{
    int numbers[] = { -1, 0, 1, 2, 4 };
    int expected = 4;
    Test("Test3", numbers, sizeof(numbers) / sizeof(int), expected);
}

void Test4()
{
    int numbers[] = { -1, 0, 1, 2, 5 };
    int expected = -1;
    Test("Test4", numbers, sizeof(numbers) / sizeof(int), expected);
}

void Test5()
{
    int numbers[] = { 0 };
    int expected = 0;
    Test("Test5", numbers, sizeof(numbers) / sizeof(int), expected);
}

void Test6()
{
    int numbers[] = { 10 };
    int expected = -1;
    Test("Test6", numbers, sizeof(numbers) / sizeof(int), expected);
}

void Test7()
{
    Test("Test7", nullptr, 0, -1);
}

int main(int argc, char* argv[])
{
    Test1();
    Test2();
    Test3();
    Test4();
    Test5();
    Test6();
    Test7();
    system("pause");
    return 0;
}