633. Sum of Square Numbers

描述

Given a non-negative integer c, your task is to decide whether there're two integers a and b such that a2 + b2 = c.

Example 1:

Input: 5
Output: True
Explanation: 1 * 1 + 2 * 2 = 5

Example 2:

Input: 3
Output: False

解法

两根指针

class Solution:
    def judgeSquareSum(self, c: int) -> bool:
        start = 0
        end = int(c ** 0.5)  # must convert to integer
        while start <= end:  # for test 0, 1, 2
            sum = start ** 2 + end ** 2  # time consuming, do not judge it in if
            if sum == c:
                return True
            elif sum > c:
                end -= 1
            else:
                start += 1
        return False

使用集合

class Solution:
    def judgeSquareSum(self, c: int) -> bool:
        hash_table = set()

        for i in range(0, int(c ** 0.5) + 1):
            hash_table.add(i ** 2)

        for each in hash_table:
            if c - each in hash_table:
                return True

        return False