633. 平方数之和

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leading to the advanced question

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方案一: 开根

var judgeSquareSum = function(c) {
    for (let a = 0; a * a <= c; a++) {
        const b = Math.sqrt(c - a * a);
        if (b === parseInt(b)) {
            return true;
        }
    }
    return false;
};

方案二: 对撞指针

var judgeSquareSum = function(c) {
    let left = 0;
    let right = Math.floor(Math.sqrt(c));
    while (left <= right) {
        const sum = left * left + right * right;
        if (sum === c) {
            return true;
        } else if (sum > c) {
            right--;
        } else {
            left++;
        }
    }
    return false;
};