合并两个有序的数列

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n=10 => 1+2+3+...+10

算法 1

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public int sum(int n) {
    int sum = 0;
    for (int i = 0; i <= n; i++) {
        sum += i;
    }
    return sum;
}

算法 2

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public int sum(int n) {
    return n * (n + 1) / 2;
}

Merge two sorted lists

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a=[1,3,5,7,9]
b=[2,4,6,8]
ans=[1,2,3,4,5,6,7,8,9]

算法 1

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public int[] merge(int[] a, int[] b) {
    int[] c = new int[a.length + b.length];
    System.arraycopy(a, 0, c, 0, a.length);
    System.arraycopy(b, 0, c, a.length, b.length);
    Arrays.sort(c);
    return c;
}

时间复杂度 O(n*log n)

算法 2

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public int[] merge(int[] a, int[] b) {
    int[] c = new int[a.length + b.length];
    int i = 0, j = 0, la = a.length, lb = b.length;
    int p = 0;
    while (i < la && j < lb) {
        if (a[i] < b[j]) {
            c[p++] = a[i];
            i++;
        } else {
            c[p++] = b[j];
            j++;
        }
    }
    while (i < la) {
        c[p++] = a[i];
        i++;
    }
    while (j < lb) {
        c[p++] = b[j];
        j++;
    }
    return c;
}

时间复杂度 O(n)