41 缺失的第一个正数

class Solution {
    public int firstMissingPositive(int[] nums) {
        Map<Integer, Integer> map = new HashMap<>();
        for (int i : nums) {
            map.put(i, -1);
        }
        int res = 1;
        while (map.containsKey(res)) {
            res++;
        }
        return res;
    }
}

42 接雨水

class Solution {
     public int trap(int[] height) {
        int res = 0;
        // 循环每一个边，往两边进行延伸，两边都能找到较大的高度，减自己的高度就代表当前节点所能有的最高高度。
       for (int i = 1; i < height.length - 1; i++) {
           int leftMax = 0;
           int rightMax = 0;

           for (int j = i; j >= 0; j--) {
               leftMax = Math.max(leftMax, height[j]);
           } 
           for (int j = i; j < height.length; j++) {
               rightMax = Math.max(rightMax, height[j]);
           }

           res += Math.min(leftMax, rightMax) - height[i];


       }
       return res;
    }
}

43 字符串相乘

class Solution {
    public String multiply(String num1, String num2) {
       int count1 = num1.length();
       int count2 = num2.length();
       int[] a = new int[count1];
       int[] b = new int[count2];
       for (int i = count1 - 1, j = 0; i >= 0; i--,j++) {
           a[j] = num1.charAt(i) - '0';
       }
       for (int i = count2 - 1, j = 0; i >= 0; i--,j++) {
           b[j] = num2.charAt(i) - '0';
       }
        // 模拟运算  a[i] * b[j] 加到 c[i + j]上 ，最后再将这些数去摸相加处理
       int[] res = new int[count2 + count1 + 1];
       for (int i = 0; i < count1; i++) {
           for (int j = 0; j < count2; j++) {
               res[i + j] += a[i] * b[j];
           }
       }

       for(int i = 0, t = 0; i < count2 + count1; i++) {
           t += res[i];
           res[i] = t % 10;
           t = t / 10;
       }
       int k = res.length - 1;
       while (k > 0 && res[k] == 0) {
           k--;
       }
       StringBuilder builder = new StringBuilder();
       for (int i = k; i>= 0; i--) {
           builder.append(res[i]);
       }
       return builder.toString();


    }
}