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29. 两数相除

结果肯定落在dividend上，于是对这个区间每一个数进行二分查找，判断方法就是

while (l < r) {long mid = l + r + 1 >> 1;if (mul(mid, y) <= x) {l = mid;} else {r = mid - 1;}}

其中mul是一个要定义的快速乘法。

完整代码如下

class Solution {public int divide(int a, int b) {long x = a, y = b;boolean isNeg = false;if ((x > 0 && y < 0) || (x < 0 && y > 0)) isNeg = true;if (x < 0) x = -x;if (y < 0) y = -y;long l = 0, r = x;while (l < r) {long mid = l + r + 1 >> 1;if (mul(mid, y) <= x) {l = mid;} else {r = mid - 1;}}long ans = isNeg ? -l : l;if (ans > Integer.MAX_VALUE || ans < Integer.MIN_VALUE) return Integer.MAX_VALUE;return (int)ans;}long mul(long a, long k) {long ans = 0;while (k > 0) {if ((k & 1) == 1) ans += a;k >>= 1;a += a;}return ans;}
}