一、二分查找

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

二、两数之和、三数之和

给你一个包含 n 个整数的数组 nums，判断 nums 中是否存在三个元素 a，b，c ，使得 a + b + c = 0 ？请你找出所有和为 0 且不重复的三元组。

// 两束之和
public int[] twoSum(int[] nums, int target) {
        int[] two = new int[2];
        for (int i = 0; i < nums.length; i++) {
            for (int j = 0; j < nums.length; j++) {
                if (nums[i]+nums[j] == target){
                    two[0] = nums[i];
                    two[1] = nums[j];
                }
            }
        }
        return two;
    }
}

// 三数之和为0
class Solution {
    public List<List<Integer>> threeSum(int[] nums) {
        Arrays.sort(nums);
        Set<List<Integer>> set = new HashSet<>();
        for(int i=0;i<nums.length;i++){

            if(nums[i]>0)break;
            int target = -nums[i];
            int left = i+1;
            int right = nums.length-1;

            while(left < right){
                if(target < nums[left] + nums[right]){
                    right--;
                }else if(target > nums[left] + nums[right]){
                    left++;
                }else{
                    set.add(Arrays.asList(nums[i], nums[left], nums[right]));
                    left++;
                }
            }
        }
        return new ArrayList<>(set);
    }
}

三、给定一个数，要求输出对应位数的n进制的数

两种方法

// Integer.toString(num, 7)
public String convertToBase7(int num, n) {
    return Integer.toString(num, n); 
}

// StringBuilder  加入 为7进制
public String convertToBase7(int num) {
        StringBuilder sb = new StringBuilder();
        if (num == 0) return "0";
        boolean flag = num < 0;
        num = Math.abs(num);
        while (num != 0) {
            sb.append(num % 7);
            num /= 7;
        }
        if (flag) sb.append("-");
        return sb.reverse().toString();
    }