二分查找 while & 递归

简介

二分查找算是 一种分治思想的体现,能在最坏时间复杂度O(logn)下查找出元素,但是,有一个重要的前提就是要提前排好序

二分查找复杂度

O(logn)

while版
public int binarySearch(int[] nums, int key) {
        if (nums.length < 1 || key < nums[0] || key > nums[nums.length - 1]) {
            return -1;
        }
        int start = 0;
        int end = nums.length - 1;
        while (start <= end) {
            int mid = (start + end) / 2;
            if (key < nums[mid]) {
                end = mid - 1;
            } else if (key > nums[mid]) {
                start = mid + 1;
            } else {
                return mid;
            }
        }
        return -1;
}
递归版
public int binarySearch(int[] nums, int key, int l, int r) {
        int n = nums.length;
        if (n < 1 || key < nums[0] || key > nums[n - 1]) {
            return -1;
        }
        int mid = (l + r) / 2;
        if (nums[mid] == key) {
            return mid;
        } else if (nums[mid] > key) {
            binarySearch(nums, key, l, mid - 1);
        } else {
            binarySearch(nums, key, l + 1, r);
        }
        return -1;
}
原文地址:https://www.cnblogs.com/worldline/p/15556173.html