[leetcode] 29. divide two integers

这道题目一直不会做，因为要考虑的corner case 太多。

1. divisor equals 0.

2. dividend equals 0.

3. Is the result negative?

4. when dividend equals Integer.MIN_VALUE and divisor equals -1, the result will overflow. convert result to long and then to integer.

5. have to use divided by divisor * 2 ^ n to avoid exceeding time limit

6. have to convert divisor and dividend to long to avoid overflow in shift.

7. constant 1 in java is compiled as integer by default.

public class Solution {
    public int divide(int dividend, int divisor) {
        if (divisor == 0) {
            return dividend >= 0 ? Integer.MAX_VALUE : Integer.MIN_VALUE;
        }
        if (dividend == 0) {
            return 0;
        }
        boolean isNegative = (dividend > 0 && divisor < 0) || (dividend < 0 && divisor > 0);
        long x = Math.abs((long)dividend);
        long y = Math.abs((long)divisor);
        long result = 0;
        while (x >= y) {
            int a = 0;
            while (x >= (y << a)) {
                a++;
            }
            a--;
            result += (long)1 << a;
            x -= y << a;
        }
        if (result == (long) Integer.MAX_VALUE + 1) {
            return isNegative ? Integer.MIN_VALUE : Integer.MAX_VALUE;
        }
        return isNegative ? -(int)result : (int)result;
    }
}