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();
}
