2021-05-04：给定一个非负整数c，你要判断是否存在两个整数a和b，使得a*a+b*b=c。

package main

import (
    "fmt"
    "math"
)

func main() {
    for i := 1000; i <= 1100; i++ {
        ret := isSquares(i)
        fmt.Println(i, ret)
    }
}

//4+1+1+1
func isSquares(n int) bool {
    return numSquares(n) <= 2
}

func numSquares(n int) int {
    for n&3 == 0 { //n%4==0
        n >>= 2 //n/=4
    }
    if n&7 == 7 { //n%8==7
        return 4
    }
    a := 0
    for a*a <= n {
        b := int(math.Sqrt(float64(n - a*a)))
        if a > b {
            break
        }
        if a*a+b*b == n {
            ret := 0
            if a != 0 {
                ret++
            }
            if b != 0 {
                ret++
            }
            return ret
        }
        a += 1
    }
    return 3
}