前端js实现各种算法，记得点赞~

题目：给定不同面额的硬币 coins 和一个总金额 amount。编写一个函数来计算可以凑成总金额所需的最少的硬币个数。

var coinChange = function (coins, amount) {
    let max = amount + 1;
    let dp = new Array(amount + 1);
    dp.fill(max);
    dp[0] = 0;
    for (let i = 1; i < max; i++) {
        for (let j = 0; j < coins.length; j++) {
            if (coins[j] <= i) {
                dp[i] = Math.min(dp[i], dp[i - coins[j]] + 1);
            }
        }
    }
    return dp[amount] > amount ? -1 : dp[amount];
};

题目：找到给定字符串中某段最长的回文。

var longestPalindrome = function (s) {
    let maxLength = 0, left = 0, right = 0
    for (let i = 0; i < s.length; i++) {
        let singleCharLength = getPalLenByCenterChar(s, i, i)
        let doubleCharLength = getPalLenByCenterChar(s, i, i + 1)
        let max = Math.max(singleCharLength, doubleCharLength)
        if (max > maxLength) {
            maxLength = max
            left = i - parseInt((max - 1) / 2)
            right = i + parseInt(max / 2)
        }
    }
    return s.slice(left, right + 1)
};

function getPalLenByCenterChar(s, left, right) {
    // 中间值为两个字符，确保两个字符相等
    if (s[left] != s[right]){
        return right - left
    }
    while (left > 0 && right < s.length - 1) {
        left--
        right++
        if (s[left] != s[right]){
            return right - left - 1
        }
    }
    return right - left + 1
}

题目：求 x 的平方根。

var mySqrt = function (x) {
    let l = 0, r = x
    while (true) {
        let mid = parseInt(l + (r - l) / 2)
        if (mid * mid > x) {
            r = mid - 1
        } else if (mid * mid < x) {
            if ((mid + 1) * (mid + 1) > x) {
                return mid
            }
            l = mid + 1
        } else {
            return mid
        }
    }
};

****题目：背包问题（动态规划）。

function knapsack (capacity, objectArr) {
    var n = objectArr.length;
    var f = [];
    for (var w = 0; w <= capacity; w++) {
        for (var i = 0; i < n; i++) {
            if (w === 0) {
                f[w] = 0;
            } else if (objectArr[i].size <= w) {
                var size = objectArr[i].size,
                    value = objectArr[i].value
                f[w] = Math.max(f[w - size] + value, f[w] || 0);
            } else {
                f[w] = Math.max(f[w] || 0, f[w - 1]);
            }
        }
    }
    return f[capacity];
} 

********题目：背包问题（贪心算法）。

function knapsack (capacity, objectArr) {
    // 首先按性价比排序, 高 -> 低
    objectArr.sort(function (a, b) {
        return parseFloat(b.value / b.size) - parseFloat(a.value / a.size);
    });
    // 记录物品个数
    var n = objectArr.length;
    // 记录已经选中尺寸，已选最大的最大价值
    var selected = 0,
        maxValue = 0;
    for (var i = 0; i < n && selected < capacity; i++) {
        var size = objectArr[i].size,
            value = objectArr[i].value;
        if (size <= capacity - selected) {
            maxValue += value;
            selected += size;
        } else {
            // 计算比例
            maxValue += value * ((capacity - selected) / size);
            selected  = capacity;
        }
    }
    return maxValue;
}

题目：统计字符串中出现次数最多的字母。

function findMaxDuplicateChar(str) {
  if(str.length == 1) {
    return str;
  }
  var charObj = {};
  for(var i = 0; i < str.length; i++) {
    if(!charObj[str.charAt(i)]) {
      charObj[str.charAt(i)] = 1;
    } else {
      charObj[str.charAt(i)] += 1;
    }
  }
  var maxChar = '',
      maxValue = 1;
  for(var k in charObj) {
    if(charObj[k] >= maxValue) {
      maxChar = k;
      maxValue = charObj[k];
    }
  }
  return maxChar + '：' + maxValue;
}