69. Sqrt(x)

Description

Given a non-negative integer x, return the square root of x rounded down to the nearest integer. The returned integer should be non-negative as well.

You must not use any built-in exponent function or operator.

  • For example, do not use pow(x, 0.5) in c++ or x ** 0.5 in python.

 

Example 1:

Input: x = 4
Output: 2
Explanation: The square root of 4 is 2, so we return 2.

Example 2:

Input: x = 8
Output: 2
Explanation: The square root of 8 is 2.82842..., and since we round it down to the nearest integer, 2 is returned.

 

Constraints:

  • 0 <= x <= 231 - 1

Solutions

We define the left boundary of the binary search as $l = 0$ and the right boundary as $r = x$, then we search for the square root within the range $[l, r]$.

In each step of the search, we find the middle value $mid = (l + r + 1) / 2$. If $mid > x / mid$, it means the square root is within the range $[l, mid - 1]$, so we set $r = mid - 1$. Otherwise, it means the square root is within the range $[mid, r]$, so we set $l = mid$.

After the search ends, we return $l$.

The time complexity is $O(\log x)$, and the space complexity is $O(1)$.

Python Code
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class Solution:
    def mySqrt(self, x: int) -> int:
        l, r = 0, x
        while l < r:
            mid = (l + r + 1) >> 1
            if mid > x // mid:
                r = mid - 1
            else:
                l = mid
        return l

Java Code
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class Solution {
    public int mySqrt(int x) {
        int l = 0, r = x;
        while (l < r) {
            int mid = (l + r + 1) >>> 1;
            if (mid > x / mid) {
                r = mid - 1;
            } else {
                l = mid;
            }
        }
        return l;
    }
}

C++ Code
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class Solution {
public:
    int mySqrt(int x) {
        int l = 0, r = x;
        while (l < r) {
            int mid = (l + r + 1ll) >> 1;
            if (mid > x / mid) {
                r = mid - 1;
            } else {
                l = mid;
            }
        }
        return l;
    }
};

Go Code
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func mySqrt(x int) int {
	return sort.Search(x+1, func(i int) bool { return i*i > x }) - 1
}

Rust Code
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impl Solution {
    pub fn my_sqrt(x: i32) -> i32 {
        let mut l = 0;
        let mut r = x;

        while l < r {
            let mid = (l + r + 1) / 2;

            if mid > x / mid {
                r = mid - 1;
            } else {
                l = mid;
            }
        }

        l
    }
}

JavaScript Code
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/**
 * @param {number} x
 * @return {number}
 */
var mySqrt = function (x) {
    let [l, r] = [0, x];
    while (l < r) {
        const mid = (l + r + 1) >> 1;
        if (mid > x / mid) {
            r = mid - 1;
        } else {
            l = mid;
        }
    }
    return l;
};

C# Code
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public class Solution {
    public int MySqrt(int x) {
        int l = 0, r = x;
        while (l < r) {
            int mid = (l + r + 1) >>> 1;
            if (mid > x / mid) {
                r = mid - 1;
            } else {
                l = mid;
            }
        }
        return l;
    }
}