[LeetCode Hot 100] LeetCode34.在排序数组中查找元素的第一个和最后一个位置

题目描述

思路：

二分查找之寻找左右侧边界
两个关键点：1. 数组有序；2. 时间复杂度O(log n)

方法一：

class Solution {
    public int[] searchRange(int[] nums, int target) {
        if (nums.length == 0 || nums == null) {
            return new int[]{-1, -1};
        }
        int leftBound = -1;
        // 寻找最左边界
        int left = 0, right = nums.length - 1;
        while (left <= right) {
            int mid = left + (right - left) / 2;
            if (target == nums[mid]) {
                if (mid == 0 || nums[mid - 1] != target) {
                    leftBound = mid;
                    break;
                } else {
                    right = mid - 1;
                }
            } else if (target < nums[mid]) {
                right = mid - 1;
            } else if (target > nums[mid]) {
                left = mid + 1;
            }
        }
		// 说明没有找到左边界,此时直接返回[-1, -1]
        if (leftBound == -1) {
            return new int[]{-1, -1};
        }
		// 开始寻找右边界
        left = 0;
        right = nums.length - 1;
        while (left <= right) {
            int mid = left + (right - left) / 2;
            if (target == nums[mid]) {
               if (mid == nums.length - 1 || nums[mid + 1] != target) {
                   return new int[]{leftBound, mid};
               } else {
                   left = mid + 1;
               }
            } else if (target < nums[mid]) {
               right = mid - 1;

            } else if (target > nums[mid]) {
                left = mid + 1;
            }
        }
        return new int[]{-1, -1};
    }
}