力扣---2020.3.9

121. 买卖股票的最佳时机

//暴力
class Solution {
    public int maxProfit(int[] prices) {
        int max = 0;
        for(int i=0;i<prices.length;i++){
            for(int j=i+1;j<prices.length;j++){
                max = Math.max(max,prices[j]-prices[i]);
            }
        }
        return max;
    }
}

class Solution {
    public int maxProfit(int[] prices) {
        int min = Integer.MAX_VALUE;
        int profit = 0;
        for(int i=0;i<prices.length;i++){
            min = Math.min(min,prices[i]);
            profit = Math.max(profit,prices[i]-min);
        }
        return profit;
    }
}

class Solution {
    public int maxProfit(int[] prices) {
        int length = prices.length;
        if (length == 0) {
            return 0;
        }
        int maxprice = 0, maxprofit = -1;
        for (int i = length - 1; i >= 0; i --) {
            if (prices[i] > maxprice) {
                maxprice = prices[i];
            }
            int profit = maxprice - prices[i];
            if (profit > maxprofit) {
                maxprofit = profit;
            }
        }
        return maxprofit;
    }
}

1365. 有多少小于当前数字的数字

class Solution {
    public int[] smallerNumbersThanCurrent(int[] nums) {
        int[] res = new int[nums.length];
        for(int i = 0;i<nums.length;i++){
            int count = 0;
            for(int j = 0;j<nums.length;j++){
                if(nums[i]>nums[j]){
                    count++;
                }
            }
            res[i] = count;
        }
        return res;
    }
}

class Solution {
    public int[] smallerNumbersThanCurrent(int[] nums) {
        int n = nums.length;
        int[] temp = new int[n];
        temp = Arrays.copyOf(nums, n);
        Arrays.sort(temp);
        Map<Integer, Integer> map = new HashMap<>();
        
        for(int i = 0; i < n; i++){
            if(i == 0){
                map.put(temp[i],0);
            }else{
                if(temp[i] > temp[i-1]){
                    map.put(temp[i],i);
                }else{
                    map.put(temp[i],map.get(temp[i-1]));
                }
            }
        }
        for(int i = 0; i < n; i++){
            temp[i] = map.get(nums[i]);
        }
        return temp;
    }
}

class Solution {
    public int[] smallerNumbersThanCurrent(int[] nums) { // 8, 1, 2, 2, 3
        int len = nums.length;
        Map<Integer, Set<Integer>> valueIndex = new HashMap<>(len); // 预存每个值与索引对应
        for (int i = 0; i < len; i++) {
            if (!valueIndex.containsKey(nums[i])) valueIndex.put(nums[i], new HashSet<>());
            valueIndex.get(nums[i]).add(i);
        }
        int[] sortedArr = Arrays.copyOf(nums, len), res = new int[len];
        Arrays.sort(sortedArr); // 1, 2, 2, 3, 8
        for (int si = len - 1; si >= 0; si--) {
            for (int i : valueIndex.get(sortedArr[si])) res[i] = si; // 同值的所有索引都更新
        }
        return res;
    }
}

1290. 二进制链表转整数

//位运算
class Solution {
    public int getDecimalValue(ListNode head) {
		if (head == null)
			return 0;
		int ans = 0;
		while (head != null) {
			ans = (ans << 1) + head.val;
			head = head.next;
		}
		return ans;
    }
}

class Solution {
    public int getDecimalValue(ListNode head) {
		if (head == null)
			return 0;
		Stack<Integer> stack = new Stack<>();
		while (head != null) {
			stack.push(head.val);
			head = head.next;
		}
		int mark = 1;
		int ans = 0;
		while (!stack.isEmpty()) {
			ans += stack.pop() * mark;
			mark <<= 1;
		}
		return ans;
    }
}

class Solution {
   public int getDecimalValue(ListNode head) {
        if(head==null){
            return 0;    
        }
        StringBuffer sb = new StringBuffer();
        while(head!=null){
            sb.append(head.val);
            head = head.next;
        }
        return  Integer.parseInt(sb.toString(),2);
    }
}

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