leetcode 633. Sum of Square Numbers

1.題目

Given a non-negative integer c, your task is to decide whether there’re two integers a and b such that a2 + b2 = c.
給一個數字C，判斷C是否能由兩個數的平方組成
Example 1:
Input: 5
Output: True
Explanation: 1 * 1 + 2 * 2 = 5
Example 2:
Input: 3
Output: False

2.分析

要找出是否存在a2 + b2 = c. 先限定a,b的範圍。 0<=a,b<=sqrt(c)
然後從兩端向中間逼近。

3.代碼

class Solution {
public:
    bool judgeSquareSum(int c) {
        int left = 0, right = sqrt(c);
        while (left <= right) {
            int result = left*left + right*right;
            if (result == c)
                return true;
            else if (result < c)
                ++left;
            else
                --right;
        }
        return false;
    }
};