二分查找學習

直接看尋找一個數、尋找左側邊界、尋找右側邊界的代碼。
即可瞭解解題基本套路。直接返回，或者鎖定邊界並且檢查越界。

int binary_search(int[] nums, int target) {
    int left = 0, right = nums.length - 1; 
    while(left <= right) {
        int mid = left + (right - left) / 2;
        if (nums[mid] < target) {
            left = mid + 1;
        } else if (nums[mid] > target) {
            right = mid - 1; 
        } else if(nums[mid] == target) {
            // 直接返回
            return mid;
        }
    }
    // 直接返回
    return -1;
}

int left_bound(int[] nums, int target) {
    int left = 0, right = nums.length - 1;
    while (left <= right) {
        int mid = left + (right - left) / 2;
        if (nums[mid] < target) {
            left = mid + 1;
        } else if (nums[mid] > target) {
            right = mid - 1;
        } else if (nums[mid] == target) {
            // 別返回，鎖定左側邊界
            right = mid - 1;
        }
    }
    // 最後要檢查 left 越界的情況
    if (left >= nums.length || nums[left] != target)
        return -1;
    return left;
}


int right_bound(int[] nums, int target) {
    int left = 0, right = nums.length - 1;
    while (left <= right) {
        int mid = left + (right - left) / 2;
        if (nums[mid] < target) {
            left = mid + 1;
        } else if (nums[mid] > target) {
            right = mid - 1;
        } else if (nums[mid] == target) {
            // 別返回，鎖定右側邊界
            left = mid + 1;
        }
    }
    // 最後要檢查 right 越界的情況
    if (right < 0 || nums[right] != target)
        return -1;
    return right;
}