代码随想录训练营第四十三天 ｜ 动态规划背包问题

 今天是第四十三天，还是背包问题

1049. 最后一块石头的重量 II 

class Solution {
    public int lastStoneWeightII(int[] stones) {
        int n = stones.length;
        int sum = 0;
        for (int i : stones) {
            sum += i;
        }
        int target = sum >> 1;
        
        int[] dp = new int[target + 1];
        for (int i = 0; i < n; i++) {
            
            for (int j = target; j >= stones[i]; j--) {
                
                dp[j] = Math.max(dp[j], dp[j - stones[i]] + stones[i]);
            }
        }
        return sum - 2 * dp[target];
    }
    
}

怎么啥都能转化成背包

494. 目标和 

class Solution {
    int res = 0;
    void backTracking(int[]nums,int target,int start,int sum){
        if(sum==target && start == nums.length){
            res++;
            return;
        }
        if(start>=nums.length)
            return;
        backTracking(nums,target,start+1,sum+nums[start]);
        backTracking(nums,target,start+1,sum-nums[start]);
    }

    public int findTargetSumWays(int[] nums, int target) {
        backTracking(nums,target,0,0);
        return res;
    }
}

动态规划没看会，回溯看会了

474.一和零 

class Solution {
    public int findMaxForm(String[] strs, int m, int n) {
        int[][] dp = new int[101][101];
        for(String s : strs){
           int a = 0;
           int b = 0;
           char[] temp = s.toCharArray();
           for(char c : temp){
               if(c == '0') a++;
               if(c == '1') b++;
           }
           for (int i = m; i >= a; i --) {
               for (int j = n; j >= b; j --) {
                   dp[i][j] = Math.max(dp[i][j], dp[i - a][j - b] + 1);
               }
           }
       }
       return dp[m][n];
    }
}

还是背包

 

今天的三道题都没太弄懂。。。。好头痛，周末好好学一下重头