202. 快乐数

方法一：用哈希集合检测循环

/*** @param {number} n* @return {boolean}*/let getNext = function(n) {return n.toString().split('').map(i => i ** 2).reduce((a, b) => a + b);
}let isHappy = function(n) {let seen = new Set();while (n !== 1 && !seen.has(n)) {seen.add(n);n = getNext(n);}return n === 1;
};

方法二：快慢指针

/*** @param {number} n* @return {boolean}*/let getNext = function(n) {return n.toString().split('').map(i => i ** 2).reduce((a, b) => a + b);
}let isHappy = function(n) {let slow = n;let fast = getNext(n);while (fast !== 1 && fast !== slow) {slow = getNext(slow);fast = getNext(getNext(fast));}return fast === 1;
};

方法三：数学

/*** @param {number} n* @return {boolean}*/
// 定义一个包含这些数字的散列集
let cycleMembers = new Set([4, 16, 37, 58, 89, 145, 42, 20]);let getNext = function(n) {return n.toString().split('').map(i => i ** 2).reduce((a, b) => a + b);
}let isHappy = function(n) {while ( n !==1 && !cycleMembers.has(n)) {n = getNext(n);}return n === 1;
};