八月份抄题

191. 位1的个数 8/4 晚11：00

思路
1.return Integer.bitCount(n);
2.https://leetcode-cn.com/problems/number-of-1-bits/solution/fu-xue-ming-zhu-xiang-jie-wei-yun-suan-f-ci7i/

n & (n - 1) 的技巧可以消除二进制中最后一个 1

public class Solution {
    // you need to treat n as an unsigned value
    public int hammingWeight(int n) {
        int res = 0;
        while (n != 0) {
            res += 1;
            n &= n - 1;
        }
        return res;
    }
}

118. 杨辉三角 8/6 下午两点二十

代码实现

class Solution {
    public List<List<Integer>> generate(int numRows) {
        List<List<Integer>> ret = new ArrayList<List<Integer>>();
        for (int i = 0; i < numRows; ++i) {
            List<Integer> row = new ArrayList<Integer>();
            for (int j = 0; j <= i; ++j) {
                if (j == 0 || j == i) {
                    row.add(1);
                } else {
                    row.add(ret.get(i - 1).get(j - 1) + ret.get(i - 1).get(j));
                }
            }
            ret.add(row);
        }
        return ret;
    }
}

股票问题：

https://leetcode-cn.com/problems/best-time-to-buy-and-sell-stock-ii/solution/tan-xin-suan-fa-by-liweiwei1419-2/

岛屿问题

前缀和问题

https://leetcode-cn.com/problems/subarray-sum-equals-k/solution/de-liao-yi-wen-jiang-qian-zhui-he-an-pai-yhyf/

前缀和思想和滑动窗口会经常用在求子数组和子串问题上