刷题常用的代码段_ocs题库配置代码

刷题常用的代码段

1. 二分法

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

while(num != 0){
   
	cou++;
	num /= 10;
}
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3.一些函数用法

//返回s的第0 - n的字符串（不包括n）
str.substring(0,n);
//将栈的值从栈底依次取出 sb为StringBuilder
for(char c : stack){
   
	sb.append(c);
}
//切断链表 将pre.next-rightnode从链表中截取
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4.计算二进制的1

//只考虑x > 0
public int countOnes(int x) {
   
	int ones = 0;
	while(x > 0){
   
		x &= (x - 1);
		ones++;
	}
	return ones;
}
//考虑有负数的情况
public int countOnes(int x) {
   
	int ones = 0;
	int flag = 1;
	while(flag != 0) {
   
		if((flag & n) != 0) ones++;
		flag = flag << 1;
	}
	return ones;
}
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5.反转链表

//反转全部
public ListNode reverse(ListNode head){
   
	ListNode pre =null;
	while(head != null){
   
		ListNode next = head.next;
		head.next = pre;
		pre = head;
		head = next;
	}
	return pre;
}
//反转部分
public void reverse(ListNode head, ListNode tail) {
   
	ListNode p1 = head, p2 = head.next, p = head, last = tail.next;
     while (p2 != last && p2 != null) {
   
	 	p1.next = p2.next;
		p2.next = p;
		p = p2;
		p2 = p1.next;
     }
}
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6.BFS & DFS

#1. BFS

public List<List<Integer>> levelOrder(TreeNode root) {
   
    //边界条件判断
    if (root == null)
        return new ArrayList<>();
    //队列
    Queue<TreeNode> queue = new LinkedList<>();
    List<List<Integer>> res = new ArrayList<>();
    //根节点入队
    queue.add(root);
    //如果队列不为空就继续循环
    while (!queue.isEmpty()) {
   
        //BFS打印，levelNum表示的是每层的结点数
        int levelNum = queue.size();
        //subList存储的是每层的结点值
        List<Integer> subList = new ArrayList<>();
        for (int i = 
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