发现算法之美-双指针之对撞指针

let arr = [1, 7, 5, 2];
let head = 0;
let tail = arr.length -1;

var arr = new Array(10).fill(0).map((num,i)=>num+i);
var i =0;
var j = arr.length - 1;
while(i<j){
  i++;
  j--;
}
var collision = [i - 1, j + 1]
var tip = `Array's head and tail had a collision between ${collision[0]} and ${collision[1]}`;
console.log(tip); // Array's head and tail had a collision between 4 and 5

var reverse = function (x) {
/**
 * 解法2.指针对撞法
 * 性能：96 ms 35.38% 35.9 MB 77.35%
 */
var sign = Math.sign(x);
var arr = x.toString().split("");
//
if (sign === -1) {
  arr.shift();
}
// 指针对撞开始
var i = 0;
var j = arr.length - 1;
while (i < j) {
  swap(arr, i, j);
  i++;
  j--;
}
function swap(arr, i, j) {
  var temp = arr[i];
  arr[i] = arr[j];
  arr[j] = temp;
}
// 指针对撞结束
var rX = parseInt(arr.join(""));
var result = sign * rX;
var min = -Math.pow(2, 31);
var max = Math.pow(2, 31) - 1;
if (rX < min || rX > max) return 0;
return result;
};

var isPalindrome = function (x) {
  /**
   * 解法3：对撞指针法
   * 性能：244ms 45.5MB
   */
  var strArr = `${x}`.split("");
  var i = 0;
  var j = strArr.length - 1;
  while (i < j) {
    if (strArr[i] !== strArr[j]) {
      return false;
    }
    i++;
    j--;
  }
  return true;
};

var removeElement = function (nums, val) {
  /**
   * 解法2：对撞指针
   * 性能：64ms 33.9MB
   */
  var i = 0;
  var j = nums.length - 1;
  while (i <= j) {
    if (nums[i] === val) {
      nums.splice(i, 1);
      j--;
    } else if (nums[j] === val) {
      nums.splice(j, 1);
      j--;
    } else {
      i++;
    }
  }
  return nums.length;
};

var isPalindrome = function (s) {
  /**
   * 解法1：正则 + 对撞指针
   * 性能：76ms 89.76% 37.5MB 70.83%
   */
  var parlinDrome = s.replace(/[^\w]/g, "").replace(/_/g, "").toLowerCase();
  var strArr = parlinDrome.split("");
  var i = 0;
  var j = strArr.length - 1;
  while (i < j) {
    if (strArr[i] !== strArr[j]) {
      return false;
    }
    i++;
    j--;
  }
  return true;
};

var twoSum = function (numbers, target) {
  /**
   * 解法2：对撞指针
   * 性能：68ms 71.18% 35.2MB 76.60%
   * 时间复杂度：O(n^2)
   */
  var left = 0;
  var right = numbers.length - 1;
  while (left < right) {
    if (numbers[left] + numbers[right] === target) {
      return [left + 1, right + 1];
    } else if (numbers[left] + numbers[right] > target) {
      right--;
    } else {
      left++;
    }
  }
};

/**
 * @param {number} n - a positive integer
 * @return {number} - a positive integer
 */
var reverseBits = function (n) {
  /**
   * 解法1：转数组后对撞交换位置
   * 性能：76ms 35.8MB
   * 思路：
   * 十进制转二进制：toString(2)
   * 32位空位补0：padStart(32,0)
   * 反转算法：对撞指针法
   * 二进制转十进制：parseInt(,2)
   */
  let arr = n.toString(2).padStart(32, 0).split("");
  let head = 0;
  let tail = arr.length - 1;
  while (head < tail) {
    swap(arr, head, tail);
    head++;
    tail--;
  }
  function swap(arr, i, j) {
    let temp = arr[i];
    arr[i] = arr[j];
    arr[j] = temp;
  }
  let result = parseInt(arr.join(""), 2);
  return result;
};

var reverseString = function (s) {
  /**
   * 解法2：对撞指针
   */
  var headIdx = 0;
  var tailIdx = s.length - 1;
  while (headIdx < tailIdx) {
    swap(s, headIdx, tailIdx);
    headIdx++;
    tailIdx--;
  }
  function swap(arr, i, j) {
    var temp = arr[i];
    arr[i] = arr[j];
    arr[j] = temp;
  }
  return s;
};

/**
 * @param {string} s
 * @return {string}
 */
var reverseVowels = function (s) {
  /**
   * 解法1：对撞指针
   * 性能：108 ms 31.59% 38.4 MB 66.67%
   */
  var univocalic = ["a", "e", "i", "o", "u", "A", "E", "I", "O", "U"];
  var sArr = s.split("");
  var head = 0;
  var tail = sArr.length - 1;
  while (head < tail) {
    if (univocalic.includes(sArr[head]) && univocalic.includes(sArr[tail])) {
      swap(sArr, head, tail);
      head++;
      tail--;
    } else if (!univocalic.includes(sArr[head])) {
      head++;
    } else if (!univocalic.includes(sArr[tail])) {
      tail--;
    }
  }
  function swap(arr, i, j) {
    var temp = arr[i];
    arr[i] = arr[j];
    arr[j] = temp;
  }
  return sArr.join("");
};

var maxArea = function (height) {
  /**
   * 解法1：对撞指针法
   * 性能：64ms 35.6MB
   * */
  var head = 0;
  var tail = height.length - 1;
  var maxCapacity = 0;
  while (head < tail) {
      maxCapacity = Math.max(Math.min(height[head], height[tail]) * (tail - head), maxCapacity)
      if (height[head] < height[tail]) {
          head++
      } else {
          tail--;
      }
  }
  return maxCapacity;
};