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  • 二分查找及各种变体实现 hunter

    public class BinarySearch {
    
        // 二分查找
        public static int bSearch(int[] nums, int target) {
            int n = nums.length;
            int low = 0;
            int high = n - 1;
    
            while (low <= high) {
                int mid = low + ((high - low) >> 1);
                if (nums[mid] > target) {
                    high = mid - 1;
                } else if (nums[mid] < target) {
                    low = mid + 1;
                } else {
                    return mid;
                }
            }
    
            return -1;
        }
    
        // 查找第一个值等于给定值的元素
        public static int searchFirstTarget(int[] nums, int target) {
            int n = nums.length;
            int low = 0;
            int high = n - 1;
    
            while (low <= high) {
                int mid = low + ((high - low) >> 1);
                if (nums[mid] > target) {
                    high = mid - 1;
                } else if (nums[mid] < target) {
                    low = mid + 1;
                } else {
                    if (mid == 0 || nums[mid - 1] != target) {
                        return mid;
                    } else {
                        high = mid - 1;
                    }
                }
            }
    
            return -1;
        }
    
        // 查找最后一个值等于给定值的元素
        public static int searchLastTarget(int[] nums, int target) {
            int n = nums.length;
            int low = 0;
            int high = n - 1;
    
            while (low <= high) {
                int mid = low + ((high - low) >> 1);
                if (nums[mid] > target) {
                    high = mid - 1;
                } else if (nums[mid] < target) {
                    low = mid + 1;
                } else {
                    if (mid == n - 1 || nums[mid + 1] != target) {
                        return mid;
                    } else {
                        low = mid + 1;
                    }
                }
            }
    
            return -1;
        }
    
        // 查找第一个大于等于给定值的元素
        public static int searchFirstGreaterOrEqual2Target(int[] nums, int target) {
            int n = nums.length;
            int low = 0;
            int high = n - 1;
    
            while (low <= high) {
                int mid = low + ((high - low) >> 1);
                if (nums[mid] >= target) {
                    if (mid == 0 || nums[mid - 1] < target) {
                        return mid;
                    } else {
                        high = mid - 1;
                    }
                } else {
                    low = mid + 1;
                }
            }
    
            return -1;
        }
    
        // 查找第一个小于等于给定值的元素
        public static int searchLastLessOrEqual2Target(int[] nums, int target) {
            int n = nums.length;
            int low = 0;
            int high = n - 1;
    
            while (low <= high) {
                int mid = low + ((high - low) >> 1);
                if (nums[mid] <= target) {
                    if (mid == n - 1 || nums[mid + 1] > target) {
                        return mid;
                    } else {
                        low = mid + 1;
                    }
                } else {
                    high = mid - 1;
                }
            }
    
            return -1;
        }
    
        // 查找元素的第一个和最后一个位置
        public static int[] searchRange(int[] nums, int target) {
            int n = nums.length;
            int low = 0;
            int high = n - 1;
    
            int[] ans = new int[]{-1, -1};
    
            while (low <= high) {
                int mid = low + ((high - low) >> 1);
                if (nums[mid] < target) {
                    low = mid + 1;
                } else if (nums[mid] > target) {
                    high = mid - 1;
                } else {
                    int start = mid;
                    int end = mid;
                    while (start >= 0 && nums[start] == target) {
                        start--;
                    }
                    while (end <= n - 1 && nums[end] == target) {
                        end++;
                    }
                    ans[0] = start + 1;
                    ans[1] = end - 1;
                    return ans;
                }
            }
    
            return ans;
        }
    }
    
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  • 原文地址:https://www.cnblogs.com/hunter-w/p/15570328.html
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