Three ways to calculate the sum of positive integers within 100 in Java

The method is as follows:

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1. Brutal solution

Use for loop to directly solve the problem one by one, the algorithm complexity is O ( n ) O(n)O(n)

/** * 
暴力解法
 * @param startInclusive * @param endExclusive * @return */ public int sumByDirect(int startInclusive, int endExclusive){ int sum = 0; for (int i = startInclusive; i < endExclusive; i++) { sum += i; } return sum; }

2. Flow programming

is the same as the brute force solution, but uses declarations A flow programming style with less code and more readability

/** * 
流式编程
 * @param startInclusive * @param endExclusive * @return */ public int sumByStream(int startInclusive, int endExclusive){ return IntStream.range(startInclusive, endExclusive).sum(); }

3. Using the summation formula

Using the arithmetic sequence summation formula

complex The degree is O ( 1 ) O(1)O(1)

/** * 
利用求和公式
 * @param startInclusive * @param endExclusive * @return */ public int sumByFormula(int startInclusive, int endExclusive){ return ((startInclusive + endExclusive - 1) * (endExclusive - startInclusive) ) >> 1; }

Test:

@Test public void Test() { System.out.println("sumByDirect=" + sumByDirect(1, 101)); System.out.println("sumByStream=" + sumByStream(1, 101)); System.out.println("sumByFormula=" + sumByFormula(1, 101)); }

Output result:

sumByDirect=5050 sumByStream=5050 sumByFormula=5050

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