leetcode-1013

先理解题意，什么叫三等分，说明三块相等，那么问题抽象成什么了？

每一块都等于sum / 3

且只要两块相等则可以推出第三块也相等

所以求解就是 两块的值为sum / 3 且相等

func canThreePartsEqualSum(A []int) bool {
    sum := 0
    for _, v := range A {
        sum += v
    }
    if sum % 3 != 0 {
        return false
    }
    sum1, sum2 := sum / 3, sum / 3
    for i := range A {
        if sum1 != 0 {
            sum1 -= A[i]
        } else if sum2 != 0 {
            sum2 -= A[i]
            if sum2 == 0 {
                return true
            }
        }
    }
    return false
}

func Method2(A []int) bool {
    sum := 0
    for i := 0; i < len(A); i++ {
        sum += A[i]
    }

    if sum % 3 != 0 {
        return false
    }
    left := 0
    right := len(A) - 1
    leftSum := A[left]
    rightSum := A[right]
    segVal := sum/3
    
    for left + 1 < right {
        if leftSum == rightSum {
            if leftSum == segVal {
                return true
            }
        }
        if leftSum < sum/3 {
            left++
            leftSum += A[left]
        }
        if rightSum < sum/3 {
            right--
            rightSum += A[right]
        }
    }
    return false
}

end