Given an array of integers A sorted in non-decreasing order,
return an array of the squares of each number, also in sorted non-decreasing order.
// 平方根完一定是正整數 eg -5*-5 = 25
input: array 是 ascending 的
output 全部轉平方根後 ascending
Example 1:
Input: [-4,-1,0,3,10]
Output: [0,1,9,16,100]
Example 2:
Input: [-7,-3,2,3,11]
Output: [4,9,9,49,121]
Note:
1 <= A.length <= 10000
-10000 <= A[i] <= 10000
A is sorted in non-decreasing order.
*/
/**
* @param {number[]} A
* @return {number[]}
*/
var sortedSquares = function(A) {}
如何解
本來最直覺想法是用 map 都平方之後再用 sort ，但其實有效能更好解法
var sortedSquares = function(A) {
// return A.map( item => {
// return Math.pow(item, 2)
// }).sort( (a,b) => a - b)
};
console.log(sortedSquares([-7,-3,2,3,11]))
// faster than 50.52% of JavaScript online submissions
改善
因為已經排序過，就算有負數，頭尾一定是最大的，所以可以用 Two pointer 排序
var sortedSquares = function(A) {
let head = 0;
let tail = A.length - 1;
let result = [];
while(head <= tail){
if(Math.pow(A[head], 2) < (Math.pow(A[tail], 2))){
result.push(Math.pow(A[tail], 2))
tail --;
}else{
result.push(Math.pow(A[head], 2))
head ++;
}
}
// reserve() 就可以了，不需要 sort，因為上面已經排序過了
return result.reverse();
};
console.log(sortedSquares([-7,-3,2,3,11]))
// faster than 93.17% of JavaScript online submissions