二分法

O(logn)

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写法

Y总二分法

代码模板1

int bsearch_1(int l, int r)
{
    while (l < r)
    {
        int mid = (l + r)/2;
        if (check(mid)) r = mid;
        else l = mid + 1;
    }
    return l;
}

代码模板2

int bsearch_2(int l, int r)
{
    while (l < r)
    {
        int mid = ( l + r + 1 ) /2;
        if (check(mid)) l = mid;
        else r = mid - 1;
    }
    return l;
}

(版本一) 左闭右闭区间

class Solution {
    public int search(int[] nums, int target) {
        // 避免当 target 小于nums[0] nums[nums.length - 1]时多次循环运算
        if (target < nums[0] || target > nums[nums.length - 1]) {
            return -1;
        }
        int left = 0, right = nums.length - 1;
        while (left <= right) {
            int mid = left + ((right - left) >> 1);
            if (nums[mid] == target)
                return mid;
            else if (nums[mid] < target)
                left = mid + 1;
            else if (nums[mid] > target)
                right = mid - 1;
        }
        return -1;
    }
}

(版本二) 左闭右开区间

class Solution {
    public int search(int[] nums, int target) {
        int left = 0, right = nums.length;
        while (left < right) {
            int mid = left + ((right - left) >> 1);
            if (nums[mid] == target)
                return mid;
            else if (nums[mid] < target)
                left = mid + 1;
            else if (nums[mid] > target)
                right = mid;
        }
        return -1;
    }
}