Q1 :leetcode 216,
Q2:739.
Q3;513,
Q4:733,
Q5:77.
Q6:39,
Q7: 40
Q8: 264 PQ解法
Q:9 203. Remove Linked List Elements
Q10:leetcode 237

//Q39
class Solution {
    public List<List<Integer>> combinationSum(int[] candidates, int target) {
        
    List<List<Integer>> results = new ArrayList<>();
      List<Integer> oneSol = new ArrayList<>();
      dfs(0,  candidates, target, 0, results, oneSol);
      return results;
        
    }
    private void dfs(int level,int[] candidates, int remaining, int start, List<List<Integer>> results, List<Integer> oneSol) {
      if (remaining <= 0 ) {
        if (remaining == 0) {
          results.add(new ArrayList<>(oneSol));
        }
        return;
      }
      
      for (int i = start; i < candidates.length; i++) {
        oneSol.add(candidates[i]);
        dfs(level + 1,candidates, remaining - candidates[i], i, results, oneSol);
        //oneSol.remove(i); why NPE?
         oneSol.remove(oneSol.size() - 1);
      }
    }
}


//Q77
class Solution {
    public List<List<Integer>> combine(int n, int k) {
       
    List<List<Integer>> results = new ArrayList<>();
      List<Integer> oneSol = new ArrayList<>();
      dfs(0, k, n, 1, results, oneSol);
      return results;
        
    }
    private void dfs(int level, int k, int n, int start, List<List<Integer>> results, List<Integer> oneSol) {
      if (level == k ) {   
          results.add(new ArrayList<>(oneSol));
        return;
      }
      
      for (int i = start; i <= n; i++) {
        oneSol.add(i);
        dfs(level + 1, k, n, i+ 1, results, oneSol);
        oneSol.remove(oneSol.size() - 1);
      }
    }
}



//Q739
class Solution {
    public int[] dailyTemperatures(int[] temperatures) {
        if (temperatures == null || temperatures.length == 0) {
            return null;
        }
        Deque<Pair> stack = new ArrayDeque<>();
        int n = temperatures.length;
        int[] result = new int[n];
        for (int i = n - 1; i >= 0; i--) {
            
            while (!stack.isEmpty() && stack.peekFirst().temp <= temperatures[i]) {
                stack.pollFirst();
            }
            
            result[i] = stack.isEmpty() ? 0 : ( stack.peekFirst().idx - i ) ;
            stack.offerFirst(new Pair(temperatures[i], i));
        }
        return result;
    }
}

class Pair {
    int temp;
    int idx;
    
    public Pair (int temp, int idx) {
        this.temp = temp;
        this.idx = idx;
    }
}


//Q513
class Solution {
    public int findBottomLeftValue(TreeNode root) {
        return helper(root, root.val).val;
    }
    
    private Pair helper(TreeNode root, int parentVal) {  //returns the left most int at cur root
        if (root == null) {  //core: root == null what val to return 
            return new Pair(0, parentVal);
        }
        Pair left = helper(root.left, root.val);
        Pair right = helper(root.right, root.val);
        
        if (left.height >= right.height) {
            left.height++;
            return left;
        } else {
            right.height++;
            return right;
        }
        
    }
}
class Pair {
    int height;
    int val;
    
    public Pair (int height, int val){
        this.height = height;
        this.val = val;
    }
}
////Q216
class Solution {
    public List<List<Integer>> combinationSum3(int k, int n) {
      List<List<Integer>> results = new ArrayList<>();
      List<Integer> oneSol = new ArrayList<>();
      dfs(0, k, n, 1, results, oneSol);
      return results;
        
    }
    private void dfs(int level, int k, int remaining, int start, List<List<Integer>> results, List<Integer> oneSol) {
      if (level == k ) {
        if (remaining == 0) {
          results.add(new ArrayList<>(oneSol));
        }
        return;
      }
      
      for (int i = start; i <= 9; i++) {
        oneSol.add(i);
        dfs(level + 1, k, remaining - i, i + 1, results, oneSol);
        oneSol.remove(oneSol.size() - 1);
      }
    }
}


//Q:733
public int[][] floodFill(int[][] image, int sr, int sc, int newColor) {
    if (newColor == image[sr][sc]) { //if没有这个条件 会stackoverflow
              return image;
          }
       dfs(image, sr, sc, image[sr][sc], newColor);
       return image;
}

private void dfs (int[][] image, int x, int y, int oldCol, int newCol ) {
  int n = image.length;
  int m = image[0].length;
  
  if (x < 0 || x >= n || y < 0 || y >= 0) {
    return;
  }
  if (image[x][y] == oldCol) {
    image[x][y] = newCol;
    dfs(image, x + 1, y    , oldCol, newCol);
    dfs(image, x - 1, y    , oldCol, newCol);
    dfs(image, x    , y + 1, oldCol, newCol);
    dfs(image, x    , y - 1, oldCol, newCol);
  }
  return;
}



//
class Solution {
    public String longestCommonPrefix(String[] strs) {
        if (strs == null || strs.length == 0) {
            return "";
        }
        String tem = strs[0];
        for (int i = 1; i < strs.length; i++ ) {
          tem = helper(tem, strs[i]);
            if (tem.length() == 0) {
                return "";
            }
        }
        return tem;
    }
    
    private String helper(String a, String b) {
        int i = 0;
        int j = 0;
        while (i < a.length() && j < b.length()) {
            if (a.charAt(i) == b.charAt(j)) {
                i++; 
                j++;
            } else {
                return a.substring(0, i);
            }
        }
        return i == a.length() ? a : b;
    }
}